Query DSL with Reified Optics — Part 3: Extending the Expression Language

In this guide, we will extend the ZIO Blocks query DSL with an expression language that goes beyond what SchemaExpr provides out of the box. By the end, you will have an Expr ADT that adds SQL-specific predicates (IN, BETWEEN, IS NULL, LIKE), type-safe aggregate functions (COUNT, SUM, AVG), and conditional expressions (CASE WHEN) — all composable with the built-in SchemaExpr operators from Parts 1 and 2.

This is Part 3 of the Query DSL series. Part 1 covered building query expressions, and Part 2 covered translating them to SQL. Here, we handle the cases where the built-in expression language is not enough.

What we'll cover:

Why SchemaExpr is deliberately closed and what that means for extension
Designing an Expr ADT that is a superset of SchemaExpr
Translating SchemaExpr into Expr via fromSchemaExpr
Adding SQL-specific predicates: IN, BETWEEN, IS NULL, LIKE
Writing bridge extension methods for seamless SchemaExpr + Expr composition
Building a single unified SQL interpreter
Adding type-safe aggregate functions and CASE WHEN for advanced SQL generation

The Problem

The built-in SchemaExpr operators cover the fundamentals: equality, comparisons, boolean logic, arithmetic, and basic string operations. But real-world SQL requires more. Consider these common queries:

-- Membership test
SELECT * FROM products WHERE category IN ('Electronics', 'Books', 'Toys')

-- Range check
SELECT * FROM products WHERE price BETWEEN 10.0 AND 100.0

-- Null handling
SELECT * FROM products WHERE description IS NULL

-- Pattern matching with SQL wildcards
SELECT * FROM products WHERE name LIKE 'Lap%'

-- Aggregation
SELECT category, COUNT(*), AVG(price)
FROM products GROUP BY category HAVING COUNT(*) > 2

-- Conditional logic
SELECT name, CASE WHEN price > 100 THEN 'expensive' ELSE 'cheap' END AS tier
FROM products

None of these can be expressed with SchemaExpr alone. You could generate the SQL strings manually, but then you lose composability — you can no longer mix these operations with the type-safe SchemaExpr predicates from Parts 1 and 2.

SchemaExpr wraps a DynamicSchemaExpr sealed trait that you cannot add new cases to. The public API is SchemaExpr; DynamicSchemaExpr is the raw serializable AST exposed through .dynamic. Instead of extending DynamicSchemaExpr directly, we define an Expr ADT that is a superset — it includes equivalent nodes for everything SchemaExpr can express, plus our custom SQL-specific operations. A fromSchemaExpr function translates SchemaExpr values into Expr by reading the underlying DynamicSchemaExpr, enabling seamless interoperability with a single unified interpreter.

Prerequisites

This guide builds on Part 1: Expressions and Part 2: SQL Generation. You should be comfortable building SchemaExpr values and translating them to SQL.

libraryDependencies += "dev.zio" %% "zio-blocks-schema" % "0.0.41"

import zio.blocks.schema._

Domain Setup

We reuse the product catalog domain from the earlier guides:

case class Product(
  name: String,
  price: Double,
  category: String,
  inStock: Boolean,
  rating: Int
)

object Product extends CompanionOptics[Product] {
  implicit val schema: Schema[Product] = Schema.derived

  val name: Lens[Product, String]     = optic(_.name)
  val price: Lens[Product, Double]    = optic(_.price)
  val category: Lens[Product, String] = optic(_.category)
  val inStock: Lens[Product, Boolean] = optic(_.inStock)
  val rating: Lens[Product, Int]      = optic(_.rating)
}

Designing the Expr ADT

The key insight is the translation pattern: keep your public extension API typed, then provide a fromSchemaExpr function that lifts any built-in SchemaExpr into your extended ADT. Ordinary application code stays on SchemaExpr, Optic, and typed constructors. Only the interpreter internals need to inspect .dynamic.

  Built-in API                               Your extension layer
┌───────────────────────────────┐       ┌───────────────────────────────────────┐
│  SchemaExpr[S, A]             │       │  Expr[S, A]                           │
│  built with optics/operators  │──────▶│  ├── Builtin(schemaExpr)              │
└───────────────────────────────┘       │  ├── Column(optic)                    │
         fromSchemaExpr ────────────────│  ├── Lit(value, schema)               │
                                        │  ├── In(expr, values, schema)         │
                                        │  ├── Between(expr, low, high, schema) │
                                        │  ├── IsNull(expr)                     │
                                        │  ├── Like(expr, pattern)              │
                                        │  ├── Agg(function, expr)              │
                                        │  └── CaseWhen(branches, else)         │
                                        └───────────────────────────────────────┘

The Expr ADT keeps the public surface typed. It can wrap any built-in SchemaExpr through Builtin, and it adds SQL-specific nodes on top. The interpreter is still free to inspect schemaExpr.dynamic internally when it needs to translate the built-in pieces.

Here is the full Expr ADT with its supporting types:

sealed trait Expr[S, A]

object Expr {

  // --- Typed public nodes ---
  final case class Builtin[S, A](schemaExpr: SchemaExpr[S, A]) extends Expr[S, A]
  final case class Column[S, A](optic: Optic[S, A]) extends Expr[S, A]
  final case class Lit[S, A](value: A, schema: Schema[A]) extends Expr[S, A]

  // Relational
  final case class Relational[S, A](left: Expr[S, A], right: Expr[S, A], op: RelOp) extends Expr[S, Boolean]

  // Logical
  final case class And[S](left: Expr[S, Boolean], right: Expr[S, Boolean]) extends Expr[S, Boolean]
  final case class Or[S](left: Expr[S, Boolean], right: Expr[S, Boolean]) extends Expr[S, Boolean]
  final case class Not[S](expr: Expr[S, Boolean]) extends Expr[S, Boolean]

  // Arithmetic
  final case class Arithmetic[S, A](left: Expr[S, A], right: Expr[S, A], op: ArithOp) extends Expr[S, A]

  // String
  final case class StringConcat[S](left: Expr[S, String], right: Expr[S, String]) extends Expr[S, String]
  final case class StringRegexMatch[S](regex: Expr[S, String], string: Expr[S, String]) extends Expr[S, Boolean]
  final case class StringLength[S](string: Expr[S, String]) extends Expr[S, Int]

  // --- SQL-specific extensions (no SchemaExpr equivalents) ---
  final case class In[S, A](expr: Expr[S, A], values: List[A], schema: Schema[A]) extends Expr[S, Boolean]
  final case class Between[S, A](expr: Expr[S, A], low: A, high: A, schema: Schema[A]) extends Expr[S, Boolean]
  final case class IsNull[S, A](expr: Expr[S, A]) extends Expr[S, Boolean]
  final case class Like[S](expr: Expr[S, String], pattern: String) extends Expr[S, Boolean]

  // --- Aggregates ---
  final case class Agg[S, A, B](function: AggFunction[A, B], expr: Expr[S, A]) extends Expr[S, B]

  // --- Conditional ---
  final case class CaseWhen[S, A](
    branches: List[(Expr[S, Boolean], Expr[S, A])],
    otherwise: Option[Expr[S, A]]
  ) extends Expr[S, A]

  // --- Factory methods ---
  def col[S, A](optic: Optic[S, A]): Expr[S, A] = Column(optic)
  def lit[S, A](value: A)(implicit schema: Schema[A]): Expr[S, A] = Lit(value, schema)

  def count[S, A](expr: Expr[S, A]): Expr[S, Long]   = Agg(AggFunction.Count(), expr)
  def sum[S](expr: Expr[S, Double]): Expr[S, Double]  = Agg(AggFunction.Sum, expr)
  def avg[S](expr: Expr[S, Double]): Expr[S, Double]  = Agg(AggFunction.Avg, expr)
  def min[S, A](expr: Expr[S, A]): Expr[S, A]         = Agg(AggFunction.Min(), expr)
  def max[S, A](expr: Expr[S, A]): Expr[S, A]         = Agg(AggFunction.Max(), expr)

  def caseWhen[S, A](branches: (Expr[S, Boolean], Expr[S, A])*): CaseWhenBuilder[S, A] =
    CaseWhenBuilder(branches.toList)

  case class CaseWhenBuilder[S, A](branches: List[(Expr[S, Boolean], Expr[S, A])]) {
    def otherwise(value: Expr[S, A]): Expr[S, A] = CaseWhen(branches, Some(value))
    def end: Expr[S, A] = CaseWhen(branches, None)
  }

  // --- Translation from SchemaExpr ---
  def fromSchemaExpr[S, A](se: SchemaExpr[S, A]): Expr[S, A] = Builtin(se)
}

// --- Operators ---

sealed trait RelOp
object RelOp {
  case object Equal              extends RelOp
  case object NotEqual           extends RelOp
  case object LessThan           extends RelOp
  case object LessThanOrEqual    extends RelOp
  case object GreaterThan        extends RelOp
  case object GreaterThanOrEqual extends RelOp
}

sealed trait ArithOp
object ArithOp {
  case object Add      extends ArithOp
  case object Subtract extends ArithOp
  case object Multiply extends ArithOp
}

// Typed aggregate functions
sealed trait AggFunction[A, B] {
  def name: String
}
object AggFunction {
  case class Count[A]() extends AggFunction[A, Long]         { val name = "COUNT" }
  case object Sum       extends AggFunction[Double, Double]  { val name = "SUM" }
  case object Avg       extends AggFunction[Double, Double]  { val name = "AVG" }
  case class Min[A]()   extends AggFunction[A, A]            { val name = "MIN" }
  case class Max[A]()   extends AggFunction[A, A]            { val name = "MAX" }
}

Here are some keynotes on the design:

Type-safe aggregates — AggFunction[A, B] encodes the return type: COUNT returns Long, SUM/AVG return Double, MIN/MAX preserve the input type.
Typed public constructors — Column stores an Optic and Lit stores a typed value plus its Schema, so extension code stays on the same public abstractions as ordinary SchemaExpr code.
fromSchemaExpr — lifts a built-in SchemaExpr into the extended ADT. The dynamic representation is only consulted later by the interpreter.

Extension Methods

To make the new operations feel natural, we define implicit classes on Optic, Expr, and SchemaExpr. The bridge implicit class on SchemaExpr auto-translates at the boundary via fromSchemaExpr, so SchemaExpr and Expr values compose seamlessly with && and ||:

implicit final class OpticExprOps[S, A](private val optic: Optic[S, A]) {
  def in(values: A*)(implicit schema: Schema[A]): Expr[S, Boolean]           = Expr.In(Expr.col(optic), values.toList, schema)
  def between(low: A, high: A)(implicit schema: Schema[A]): Expr[S, Boolean] = Expr.Between(Expr.col(optic), low, high, schema)
  def isNull: Expr[S, Boolean]                   = Expr.IsNull(Expr.col(optic))
  def isNotNull: Expr[S, Boolean]                = Expr.Not(Expr.IsNull(Expr.col(optic)))
}

implicit final class StringOpticExprOps[S](private val optic: Optic[S, String]) {
  def like(pattern: String): Expr[S, Boolean] = Expr.Like(Expr.col(optic), pattern)
}

// Boolean combinators — accept both Expr and SchemaExpr on the right
implicit final class ExprBooleanOps[S](private val self: Expr[S, Boolean]) {
  def &&(other: Expr[S, Boolean]): Expr[S, Boolean]      = Expr.And(self, other)
  def &&(other: SchemaExpr[S, Boolean]): Expr[S, Boolean] = Expr.And(self, Expr.fromSchemaExpr(other))
  def ||(other: Expr[S, Boolean]): Expr[S, Boolean]      = Expr.Or(self, other)
  def ||(other: SchemaExpr[S, Boolean]): Expr[S, Boolean] = Expr.Or(self, Expr.fromSchemaExpr(other))
  def unary_! : Expr[S, Boolean]                          = Expr.Not(self)
}

// Bridge: SchemaExpr on the left, Expr on the right
implicit final class SchemaExprBooleanBridge[S](private val self: SchemaExpr[S, Boolean]) {
  def &&(other: Expr[S, Boolean]): Expr[S, Boolean] = Expr.And(Expr.fromSchemaExpr(self), other)
  def ||(other: Expr[S, Boolean]): Expr[S, Boolean] = Expr.Or(Expr.fromSchemaExpr(self), other)
  def toExpr: Expr[S, Boolean] = Expr.fromSchemaExpr(self)
}

The bridge implicit classes are the key to ergonomic composition. When you write Product.category.in("Electronics") && (Product.rating >= 4), the && on Expr[S, Boolean] sees a SchemaExpr[S, Boolean] on the right and auto-translates it. Similarly, (Product.rating >= 4) && Product.category.in("Electronics") uses the SchemaExpr bridge to translate the left side. No explicit .toExpr is needed in most cases.

tip

The .toExpr method is still available for cases where you need to explicitly lift a SchemaExpr[S, Boolean] — for example, when building CASE WHEN branch conditions.

The Unified SQL Interpreter

With the Expr ADT, we write a single interpreter that handles all cases directly. Typed extension nodes stay typed; built-in SchemaExpr fragments delegate to a helper that reads .dynamic internally:

def columnName(optic: zio.blocks.schema.Optic[_, _]): String =
  optic.toDynamic.nodes.collect { case f: DynamicOptic.Node.Field => f.name }.mkString("_")

def columnName(path: DynamicOptic): String =
  path.nodes.collect { case f: DynamicOptic.Node.Field => f.name }.mkString("_")

def sqlLiteral[A](value: A, schema: Schema[A]): String = {
  val dv = schema.toDynamicValue(value)
  dv match {
    case p: DynamicValue.Primitive => p.value match {
      case _: PrimitiveValue.String  => s"'${value.toString.replace("'", "''")}'"
      case b: PrimitiveValue.Boolean => if (b.value) "TRUE" else "FALSE"
      case _                         => value.toString
    }
    case _ => value.toString
  }
}

def sqlLiteralDV(dv: DynamicValue): String = dv match {
  case DynamicValue.Primitive(pv) =>
    pv match {
      case PrimitiveValue.String(s)  => s"'${s.replace("'", "''")}'"
      case PrimitiveValue.Boolean(b) => if (b) "TRUE" else "FALSE"
      case PrimitiveValue.Int(n)     => n.toString
      case PrimitiveValue.Long(n)    => n.toString
      case PrimitiveValue.Double(n)  => n.toString
      case PrimitiveValue.Float(n)   => n.toString
      case PrimitiveValue.Short(n)   => n.toString
      case PrimitiveValue.Byte(n)    => n.toString
      case other                     => other.toString
    }
  case other => other.toString
}

def exprToSql[S, A](expr: Expr[S, A]): String = expr match {
  case Expr.Builtin(schemaExpr) => schemaExprToSql(schemaExpr)
  case Expr.Column(optic)       => columnName(optic)
  case Expr.Lit(value, schema)  => sqlLiteral(value, schema)

  case Expr.Relational(left, right, op) =>
    val sqlOp = op match {
      case RelOp.Equal              => "="
      case RelOp.NotEqual           => "<>"
      case RelOp.LessThan           => "<"
      case RelOp.LessThanOrEqual    => "<="
      case RelOp.GreaterThan        => ">"
      case RelOp.GreaterThanOrEqual => ">="
    }
    s"(${exprToSql(left)} $sqlOp ${exprToSql(right)})"

  case Expr.And(l, r) => s"(${exprToSql(l)} AND ${exprToSql(r)})"
  case Expr.Or(l, r)  => s"(${exprToSql(l)} OR ${exprToSql(r)})"
  case Expr.Not(e)    => s"NOT (${exprToSql(e)})"

  case Expr.Arithmetic(left, right, op) =>
    val sqlOp = op match {
      case ArithOp.Add      => "+"
      case ArithOp.Subtract => "-"
      case ArithOp.Multiply => "*"
    }
    s"(${exprToSql(left)} $sqlOp ${exprToSql(right)})"

  case Expr.StringConcat(l, r)          => s"CONCAT(${exprToSql(l)}, ${exprToSql(r)})"
  case Expr.StringRegexMatch(regex, s)  => s"(${exprToSql(s)} LIKE ${exprToSql(regex)})"
  case Expr.StringLength(s)             => s"LENGTH(${exprToSql(s)})"

  // SQL-specific
  case Expr.In(e, values, schema) =>
    s"${exprToSql(e)} IN (${values.map(v => sqlLiteral(v, schema)).mkString(", ")})"
  case Expr.Between(e, low, high, schema) =>
    s"(${exprToSql(e)} BETWEEN ${sqlLiteral(low, schema)} AND ${sqlLiteral(high, schema)})"
  case Expr.IsNull(e)          => s"${exprToSql(e)} IS NULL"
  case Expr.Like(e, pattern)   => s"${exprToSql(e)} LIKE '${pattern.replace("'", "''")}'"

  // Aggregates
  case Expr.Agg(func, e) => s"${func.name}(${exprToSql(e)})"

  // CASE WHEN
  case Expr.CaseWhen(branches, otherwise) =>
    val cases = branches.map { case (cond, value) =>
      s"WHEN ${exprToSql(cond)} THEN ${exprToSql(value)}"
    }.mkString(" ")
    val elseClause = otherwise.map(e => s" ELSE ${exprToSql(e)}").getOrElse("")
    s"CASE $cases$elseClause END"
}

def schemaExprToSql[S, A](expr: SchemaExpr[S, A]): String =
  toSqlDynamic(expr.dynamic)

def toSqlDynamic(expr: DynamicSchemaExpr): String = expr match {
  case DynamicSchemaExpr.Select(path)   => columnName(path)
  case DynamicSchemaExpr.Literal(value) => sqlLiteralDV(value)

  case DynamicSchemaExpr.Relational(left, right, op) =>
    val sqlOp = op match {
      case DynamicSchemaExpr.RelationalOperator.Equal              => "="
      case DynamicSchemaExpr.RelationalOperator.NotEqual           => "<>"
      case DynamicSchemaExpr.RelationalOperator.LessThan           => "<"
      case DynamicSchemaExpr.RelationalOperator.LessThanOrEqual    => "<="
      case DynamicSchemaExpr.RelationalOperator.GreaterThan        => ">"
      case DynamicSchemaExpr.RelationalOperator.GreaterThanOrEqual => ">="
    }
    s"(${toSqlDynamic(left)} $sqlOp ${toSqlDynamic(right)})"

  case DynamicSchemaExpr.Logical(left, right, op) =>
    val sqlOp = op match {
      case DynamicSchemaExpr.LogicalOperator.And => "AND"
      case DynamicSchemaExpr.LogicalOperator.Or  => "OR"
    }
    s"(${toSqlDynamic(left)} $sqlOp ${toSqlDynamic(right)})"

  case DynamicSchemaExpr.Not(inner) =>
    s"NOT (${toSqlDynamic(inner)})"

  case DynamicSchemaExpr.Arithmetic(left, right, op, _) =>
    val sqlOp = op match {
      case DynamicSchemaExpr.ArithmeticOperator.Add      => "+"
      case DynamicSchemaExpr.ArithmeticOperator.Subtract => "-"
      case DynamicSchemaExpr.ArithmeticOperator.Multiply => "*"
      case _                                             => "?"
    }
    s"(${toSqlDynamic(left)} $sqlOp ${toSqlDynamic(right)})"

  case DynamicSchemaExpr.StringConcat(left, right) =>
    s"CONCAT(${toSqlDynamic(left)}, ${toSqlDynamic(right)})"

  case DynamicSchemaExpr.StringRegexMatch(regex, string) =>
    s"(${toSqlDynamic(string)} LIKE ${toSqlDynamic(regex)})"

  case DynamicSchemaExpr.StringLength(string) =>
    s"LENGTH(${toSqlDynamic(string)})"

  case _ => "?"
}

The typed sqlLiteral[A](value, schema) uses the Schema carried by Lit, In, and Between to format values correctly — strings get quoted, booleans become TRUE/FALSE, numbers stay as-is. sqlLiteralDV is only needed inside toSqlDynamic, where built-in SchemaExpr nodes have already crossed into the dynamic representation.

SQL-Specific Predicates

With the ADT, extensions, and interpreter in place, the new operators work directly on optics:

exprToSql(Product.category.in("Electronics", "Books", "Toys"))
// res0: String = "category IN ('Electronics', 'Books', 'Toys')"

exprToSql(Product.price.between(10.0, 100.0))
// res1: String = "(price BETWEEN 10.0 AND 100.0)"

exprToSql(Product.name.isNull)
// res2: String = "name IS NULL"

exprToSql(Product.name.isNotNull)
// res3: String = "NOT (name IS NULL)"

exprToSql(Product.name.like("Lap%"))
// res4: String = "name LIKE 'Lap%'"

Each extension method on Optic returns an Expr node. The interpreter handles it and produces the corresponding SQL fragment.

Composing with SchemaExpr

The bridge implicit classes handle the translation automatically. You can freely mix SchemaExpr predicates (from ===, >, etc.) with Expr predicates (from .in, .between, etc.) using && and ||:

// SchemaExpr values from built-in operators
val highRated: SchemaExpr[Product, Boolean] = Product.rating >= 4

// Expr values from extension methods
val inCategory: Expr[Product, Boolean] = Product.category.in("Electronics", "Books")
val priceRange: Expr[Product, Boolean] = Product.price.between(10.0, 500.0)

// Seamless composition — bridge auto-translates at the boundary
val combined: Expr[Product, Boolean] =
  inCategory && priceRange && highRated

exprToSql(combined)
// res5: String = "((category IN ('Electronics', 'Books') AND (price BETWEEN 10.0 AND 500.0)) AND (rating >= 4))"

The && between priceRange (an Expr) and highRated (a SchemaExpr) triggers the overloaded && that accepts SchemaExpr on the right. It calls fromSchemaExpr internally, so no explicit .toExpr is needed.

You can also start from a SchemaExpr on the left — the bridge implicit class handles it:

val query: Expr[Product, Boolean] =
  Product.category.in("Electronics", "Books") &&
  Product.price.between(10.0, 500.0) &&
  (Product.rating >= 4) &&
  Product.name.like("M%")

exprToSql(query)
// res6: String = "(((category IN ('Electronics', 'Books') AND (price BETWEEN 10.0 AND 500.0)) AND (rating >= 4)) AND name LIKE 'M%')"

A helper function to generate full SELECT statements from Expr predicates:

def selectWhere(table: String, predicate: Expr[_, Boolean]): String =
  s"SELECT * FROM $table WHERE ${exprToSql(predicate)}"

selectWhere("products", query)
// res7: String = "SELECT * FROM products WHERE (((category IN ('Electronics', 'Books') AND (price BETWEEN 10.0 AND 500.0)) AND (rating >= 4)) AND name LIKE 'M%')"

Aggregate Expressions

The Agg node wraps any column expression with a type-safe aggregate function. The return type reflects SQL semantics: COUNT returns Expr[S, Long], SUM/AVG return Expr[S, Double], and MIN/MAX preserve the input type:

exprToSql(Expr.count(Expr.col(Product.name)))
// res8: String = "COUNT(name)"

exprToSql(Expr.avg(Expr.col(Product.price)))
// res9: String = "AVG(price)"

exprToSql(Expr.max(Expr.col(Product.rating)))
// res10: String = "MAX(rating)"

Aggregates compose with the rest of the ADT. Build a GROUP BY query by combining aggregate SQL fragments with a select builder:

def selectGroupBy(
  table: String,
  columns: List[String],
  groupBy: List[String],
  having: Option[String] = None
): String = {
  val base = s"SELECT ${columns.mkString(", ")} FROM $table GROUP BY ${groupBy.mkString(", ")}"
  having.fold(base)(h => s"$base HAVING $h")
}

selectGroupBy(
  "products",
  columns = List(
    "category",
    s"${exprToSql(Expr.count(Expr.col(Product.name)))} AS product_count",
    s"${exprToSql(Expr.avg(Expr.col(Product.price)))} AS avg_price"
  ),
  groupBy = List("category"),
  having = Some(s"${exprToSql(Expr.count(Expr.col(Product.name)))} > 2")
)
// res11: String = "SELECT category, COUNT(name) AS product_count, AVG(price) AS avg_price FROM products GROUP BY category HAVING COUNT(name) > 2"

CASE WHEN Expressions

The CaseWhen node represents SQL's conditional expression. Use the Expr.caseWhen builder with (condition -> result) pairs and an optional .otherwise clause:

val priceLabel: Expr[Product, String] = Expr.caseWhen[Product, String](
  (Product.price > 100.0).toExpr  -> Expr.lit[Product, String]("expensive"),
  (Product.price > 10.0).toExpr   -> Expr.lit[Product, String]("moderate")
).otherwise(Expr.lit[Product, String]("cheap"))

exprToSql(priceLabel)
// res12: String = "CASE WHEN (price > 100.0) THEN 'expensive' WHEN (price > 10.0) THEN 'moderate' ELSE 'cheap' END"

CASE WHEN is useful for computed columns in SELECT lists:

val stockStatus: Expr[Product, String] = Expr.caseWhen[Product, String](
  (Product.inStock === true).toExpr -> Expr.lit[Product, String]("available")
).otherwise(Expr.lit[Product, String]("out of stock"))

val selectSql = s"SELECT name, price, ${exprToSql(priceLabel)} AS tier, ${exprToSql(stockStatus)} AS status FROM products"
// selectSql: String = "SELECT name, price, CASE WHEN (price > 100.0) THEN 'expensive' WHEN (price > 10.0) THEN 'moderate' ELSE 'cheap' END AS tier, CASE WHEN (inStock = TRUE) THEN 'available' ELSE 'out of stock' END AS status FROM products"
println(selectSql)
// SELECT name, price, CASE WHEN (price > 100.0) THEN 'expensive' WHEN (price > 10.0) THEN 'moderate' ELSE 'cheap' END AS tier, CASE WHEN (inStock = TRUE) THEN 'available' ELSE 'out of stock' END AS status FROM products

Putting It Together

Here is a complete, self-contained example that defines the independent expression ADT, lifts built-in SchemaExpr values into it, and generates advanced SQL:

import zio.blocks.schema._

// --- Domain ---

case class Product(
  name: String,
  price: Double,
  category: String,
  inStock: Boolean,
  rating: Int
)

object Product extends CompanionOptics[Product] {
  implicit val schema: Schema[Product] = Schema.derived

  val name: Lens[Product, String]     = optic(_.name)
  val price: Lens[Product, Double]    = optic(_.price)
  val category: Lens[Product, String] = optic(_.category)
  val inStock: Lens[Product, Boolean] = optic(_.inStock)
  val rating: Lens[Product, Int]      = optic(_.rating)
}

// --- Independent Expr ADT ---

sealed trait Expr[S, A]

object Expr {
  final case class Builtin[S, A](schemaExpr: SchemaExpr[S, A]) extends Expr[S, A]
  final case class Column[S, A](optic: Optic[S, A]) extends Expr[S, A]
  final case class Lit[S, A](value: A, schema: Schema[A]) extends Expr[S, A]

  final case class Relational[S, A](left: Expr[S, A], right: Expr[S, A], op: RelOp) extends Expr[S, Boolean]
  final case class And[S](left: Expr[S, Boolean], right: Expr[S, Boolean]) extends Expr[S, Boolean]
  final case class Or[S](left: Expr[S, Boolean], right: Expr[S, Boolean]) extends Expr[S, Boolean]
  final case class Not[S](expr: Expr[S, Boolean]) extends Expr[S, Boolean]
  final case class Arithmetic[S, A](left: Expr[S, A], right: Expr[S, A], op: ArithOp) extends Expr[S, A]
  final case class StringConcat[S](left: Expr[S, String], right: Expr[S, String]) extends Expr[S, String]
  final case class StringRegexMatch[S](regex: Expr[S, String], string: Expr[S, String]) extends Expr[S, Boolean]
  final case class StringLength[S](string: Expr[S, String]) extends Expr[S, Int]

  final case class In[S, A](expr: Expr[S, A], values: List[A], schema: Schema[A]) extends Expr[S, Boolean]
  final case class Between[S, A](expr: Expr[S, A], low: A, high: A, schema: Schema[A]) extends Expr[S, Boolean]
  final case class IsNull[S, A](expr: Expr[S, A]) extends Expr[S, Boolean]
  final case class Like[S](expr: Expr[S, String], pattern: String) extends Expr[S, Boolean]

  final case class Agg[S, A, B](function: AggFunction[A, B], expr: Expr[S, A]) extends Expr[S, B]
  final case class CaseWhen[S, A](
    branches: List[(Expr[S, Boolean], Expr[S, A])],
    otherwise: Option[Expr[S, A]]
  ) extends Expr[S, A]

  def col[S, A](optic: Optic[S, A]): Expr[S, A] = Column(optic)
  def lit[S, A](value: A)(implicit schema: Schema[A]): Expr[S, A] = Lit(value, schema)
  def count[S, A](expr: Expr[S, A]): Expr[S, Long]   = Agg(AggFunction.Count(), expr)
  def sum[S](expr: Expr[S, Double]): Expr[S, Double]  = Agg(AggFunction.Sum, expr)
  def avg[S](expr: Expr[S, Double]): Expr[S, Double]  = Agg(AggFunction.Avg, expr)
  def min[S, A](expr: Expr[S, A]): Expr[S, A]         = Agg(AggFunction.Min(), expr)
  def max[S, A](expr: Expr[S, A]): Expr[S, A]         = Agg(AggFunction.Max(), expr)

  def caseWhen[S, A](branches: (Expr[S, Boolean], Expr[S, A])*): CaseWhenBuilder[S, A] =
    CaseWhenBuilder(branches.toList)

  case class CaseWhenBuilder[S, A](branches: List[(Expr[S, Boolean], Expr[S, A])]) {
    def otherwise(value: Expr[S, A]): Expr[S, A] = CaseWhen(branches, Some(value))
    def end: Expr[S, A] = CaseWhen(branches, None)
  }

  def fromSchemaExpr[S, A](se: SchemaExpr[S, A]): Expr[S, A] = Builtin(se)
}

sealed trait RelOp
object RelOp {
  case object Equal              extends RelOp
  case object NotEqual           extends RelOp
  case object LessThan           extends RelOp
  case object LessThanOrEqual    extends RelOp
  case object GreaterThan        extends RelOp
  case object GreaterThanOrEqual extends RelOp
}

sealed trait ArithOp
object ArithOp {
  case object Add      extends ArithOp
  case object Subtract extends ArithOp
  case object Multiply extends ArithOp
}

sealed trait AggFunction[A, B] { def name: String }
object AggFunction {
  case class Count[A]() extends AggFunction[A, Long]        { val name = "COUNT" }
  case object Sum       extends AggFunction[Double, Double]  { val name = "SUM" }
  case object Avg       extends AggFunction[Double, Double]  { val name = "AVG" }
  case class Min[A]()   extends AggFunction[A, A]            { val name = "MIN" }
  case class Max[A]()   extends AggFunction[A, A]            { val name = "MAX" }
}

// --- Extension methods with bridge ---

implicit final class OpticExprOps[S, A](private val optic: Optic[S, A]) {
  def in(values: A*)(implicit schema: Schema[A]): Expr[S, Boolean]           = Expr.In(Expr.col(optic), values.toList, schema)
  def between(low: A, high: A)(implicit schema: Schema[A]): Expr[S, Boolean] = Expr.Between(Expr.col(optic), low, high, schema)
  def isNull: Expr[S, Boolean]                   = Expr.IsNull(Expr.col(optic))
  def isNotNull: Expr[S, Boolean]                = Expr.Not(Expr.IsNull(Expr.col(optic)))
}

implicit final class StringOpticExprOps[S](private val optic: Optic[S, String]) {
  def like(pattern: String): Expr[S, Boolean] = Expr.Like(Expr.col(optic), pattern)
}

implicit final class ExprBooleanOps[S](private val self: Expr[S, Boolean]) {
  def &&(other: Expr[S, Boolean]): Expr[S, Boolean]      = Expr.And(self, other)
  def &&(other: SchemaExpr[S, Boolean]): Expr[S, Boolean] = Expr.And(self, Expr.fromSchemaExpr(other))
  def ||(other: Expr[S, Boolean]): Expr[S, Boolean]      = Expr.Or(self, other)
  def ||(other: SchemaExpr[S, Boolean]): Expr[S, Boolean] = Expr.Or(self, Expr.fromSchemaExpr(other))
  def unary_! : Expr[S, Boolean]                          = Expr.Not(self)
}

implicit final class SchemaExprBooleanBridge[S](private val self: SchemaExpr[S, Boolean]) {
  def &&(other: Expr[S, Boolean]): Expr[S, Boolean] = Expr.And(Expr.fromSchemaExpr(self), other)
  def ||(other: Expr[S, Boolean]): Expr[S, Boolean] = Expr.Or(Expr.fromSchemaExpr(self), other)
  def toExpr: Expr[S, Boolean] = Expr.fromSchemaExpr(self)
}

// --- SQL rendering ---

def columnName(optic: zio.blocks.schema.Optic[_, _]): String =
  optic.toDynamic.nodes.collect { case f: DynamicOptic.Node.Field => f.name }.mkString("_")

def columnName(path: DynamicOptic): String =
  path.nodes.collect { case f: DynamicOptic.Node.Field => f.name }.mkString("_")

def sqlLiteral[A](value: A, schema: Schema[A]): String = {
  val dv = schema.toDynamicValue(value)
  dv match {
    case p: DynamicValue.Primitive => p.value match {
      case _: PrimitiveValue.String  => s"'${value.toString.replace("'", "''")}'"
      case b: PrimitiveValue.Boolean => if (b.value) "TRUE" else "FALSE"
      case _                         => value.toString
    }
    case _ => value.toString
  }
}

def sqlLiteralDV(dv: DynamicValue): String = dv match {
  case DynamicValue.Primitive(pv) =>
    pv match {
      case PrimitiveValue.String(s)  => s"'${s.replace("'", "''")}'"
      case PrimitiveValue.Boolean(b) => if (b) "TRUE" else "FALSE"
      case PrimitiveValue.Int(n)     => n.toString
      case PrimitiveValue.Long(n)    => n.toString
      case PrimitiveValue.Double(n)  => n.toString
      case PrimitiveValue.Float(n)   => n.toString
      case PrimitiveValue.Short(n)   => n.toString
      case PrimitiveValue.Byte(n)    => n.toString
      case other                     => other.toString
    }
  case other => other.toString
}

def exprToSql[S, A](expr: Expr[S, A]): String = expr match {
  case Expr.Builtin(schemaExpr) => schemaExprToSql(schemaExpr)
  case Expr.Column(optic)       => columnName(optic)
  case Expr.Lit(value, schema)  => sqlLiteral(value, schema)
  case Expr.Relational(left, right, op) =>
    val sqlOp = op match {
      case RelOp.Equal => "="; case RelOp.NotEqual => "<>"
      case RelOp.LessThan => "<"; case RelOp.LessThanOrEqual => "<="
      case RelOp.GreaterThan => ">"; case RelOp.GreaterThanOrEqual => ">="
    }
    s"(${exprToSql(left)} $sqlOp ${exprToSql(right)})"
  case Expr.And(l, r)                   => s"(${exprToSql(l)} AND ${exprToSql(r)})"
  case Expr.Or(l, r)                    => s"(${exprToSql(l)} OR ${exprToSql(r)})"
  case Expr.Not(e)                      => s"NOT (${exprToSql(e)})"
  case Expr.Arithmetic(left, right, op) =>
    val sqlOp = op match {
      case ArithOp.Add => "+"; case ArithOp.Subtract => "-"; case ArithOp.Multiply => "*"
    }
    s"(${exprToSql(left)} $sqlOp ${exprToSql(right)})"
  case Expr.StringConcat(l, r)         => s"CONCAT(${exprToSql(l)}, ${exprToSql(r)})"
  case Expr.StringRegexMatch(regex, s) => s"(${exprToSql(s)} LIKE ${exprToSql(regex)})"
  case Expr.StringLength(s)            => s"LENGTH(${exprToSql(s)})"
  case Expr.In(e, values, schema) =>
    s"${exprToSql(e)} IN (${values.map(v => sqlLiteral(v, schema)).mkString(", ")})"
  case Expr.Between(e, low, high, schema) =>
    s"(${exprToSql(e)} BETWEEN ${sqlLiteral(low, schema)} AND ${sqlLiteral(high, schema)})"
  case Expr.IsNull(e)        => s"${exprToSql(e)} IS NULL"
  case Expr.Like(e, pattern) => s"${exprToSql(e)} LIKE '${pattern.replace("'", "''")}'"
  case Expr.Agg(func, e)     => s"${func.name}(${exprToSql(e)})"
  case Expr.CaseWhen(branches, otherwise) =>
    val cases = branches.map { case (cond, value) =>
      s"WHEN ${exprToSql(cond)} THEN ${exprToSql(value)}"
    }.mkString(" ")
    val elseClause = otherwise.map(e => s" ELSE ${exprToSql(e)}").getOrElse("")
    s"CASE $cases$elseClause END"
}

def schemaExprToSql[S, A](expr: SchemaExpr[S, A]): String =
  toSqlDynamic(expr.dynamic)

def toSqlDynamic(expr: DynamicSchemaExpr): String = expr match {
  case DynamicSchemaExpr.Select(path)   => columnName(path)
  case DynamicSchemaExpr.Literal(value) => sqlLiteralDV(value)
  case DynamicSchemaExpr.Relational(left, right, op) =>
    val sqlOp = op match {
      case DynamicSchemaExpr.RelationalOperator.Equal              => "="
      case DynamicSchemaExpr.RelationalOperator.NotEqual           => "<>"
      case DynamicSchemaExpr.RelationalOperator.LessThan           => "<"
      case DynamicSchemaExpr.RelationalOperator.LessThanOrEqual    => "<="
      case DynamicSchemaExpr.RelationalOperator.GreaterThan        => ">"
      case DynamicSchemaExpr.RelationalOperator.GreaterThanOrEqual => ">="
    }
    s"(${toSqlDynamic(left)} $sqlOp ${toSqlDynamic(right)})"
  case DynamicSchemaExpr.Logical(left, right, op) =>
    val sqlOp = op match {
      case DynamicSchemaExpr.LogicalOperator.And => "AND"
      case DynamicSchemaExpr.LogicalOperator.Or  => "OR"
    }
    s"(${toSqlDynamic(left)} $sqlOp ${toSqlDynamic(right)})"
  case DynamicSchemaExpr.Not(inner) =>
    s"NOT (${toSqlDynamic(inner)})"
  case DynamicSchemaExpr.Arithmetic(left, right, op, _) =>
    val sqlOp = op match {
      case DynamicSchemaExpr.ArithmeticOperator.Add      => "+"
      case DynamicSchemaExpr.ArithmeticOperator.Subtract => "-"
      case DynamicSchemaExpr.ArithmeticOperator.Multiply => "*"
      case _                                             => "?"
    }
    s"(${toSqlDynamic(left)} $sqlOp ${toSqlDynamic(right)})"
  case DynamicSchemaExpr.StringConcat(left, right) =>
    s"CONCAT(${toSqlDynamic(left)}, ${toSqlDynamic(right)})"
  case DynamicSchemaExpr.StringRegexMatch(regex, string) =>
    s"(${toSqlDynamic(string)} LIKE ${toSqlDynamic(regex)})"
  case DynamicSchemaExpr.StringLength(string) =>
    s"LENGTH(${toSqlDynamic(string)})"
  case _ => "?"
}

// --- Usage ---

// 1. SQL-specific predicates — seamless composition
val q1 = Product.category.in("Electronics", "Books") &&
  Product.price.between(10.0, 500.0) &&
  (Product.rating >= 4) &&
  Product.name.like("M%")

println(s"SELECT * FROM products WHERE ${exprToSql(q1)}")

// 2. Type-safe aggregation
val countExpr: Expr[Product, Long]   = Expr.count(Expr.col(Product.name))
val avgExpr: Expr[Product, Double]   = Expr.avg(Expr.col(Product.price))
val countSql = exprToSql(countExpr)
val avgSql   = exprToSql(avgExpr)
println(s"SELECT category, $countSql AS cnt, $avgSql AS avg_price FROM products GROUP BY category HAVING $countSql > 2")

// 3. CASE WHEN
val tier = Expr.caseWhen[Product, String](
  (Product.price > 100.0).toExpr -> Expr.lit[Product, String]("expensive"),
  (Product.price > 10.0).toExpr  -> Expr.lit[Product, String]("moderate")
).otherwise(Expr.lit[Product, String]("cheap"))

println(s"SELECT name, price, ${exprToSql(tier)} AS tier FROM products")

Going Further

Part 1: Expressions -- Building query expressions with reified optics
Part 2: SQL Generation -- Translating built-in expressions to SQL
Part 4: A Fluent SQL Builder -- Type-safe SELECT, UPDATE, INSERT, DELETE with seamless condition mixing
SchemaExpr Reference -- Full API coverage of expression types
Optics Reference -- Lens, Prism, Optional, and Traversal

The translation pattern shown here extends to any domain where SchemaExpr falls short. The same approach works for MongoDB operators ($in, $exists, $elemMatch), Elasticsearch queries (terms, range, exists), or GraphQL filters. Define an independent ADT, provide a fromSchemaExpr lift for built-in expressions, add your domain-specific nodes, and let your interpreter inspect .dynamic only inside its internal translation helpers.

The Problem​

Prerequisites​

Domain Setup​

Designing the Expr ADT​

Extension Methods​

The Unified SQL Interpreter​

SQL-Specific Predicates​

Composing with SchemaExpr​

Aggregate Expressions​

CASE WHEN Expressions​

Putting It Together​

Going Further​