add case statements/operators.

src/expression/expression-builder.ts

@@ -18,8 +18,10 @@ import {
 } from '../query-builder/function-module.js'
 import {
   ExtractTypeFromReferenceExpression,
+  parseReferenceExpression,
   parseStringReference,
   ReferenceExpression,
+  SimpleReferenceExpression,
   StringReference,
 } from '../parser/reference-parser.js'
 import { QueryExecutor } from '../query-executor/query-executor.js'
@@ -47,6 +49,9 @@ import {
 } from '../parser/value-parser.js'
 import { NOOP_QUERY_EXECUTOR } from '../query-executor/noop-query-executor.js'
 import { ValueNode } from '../operation-node/value-node.js'
+import { CaseBuilder } from '../query-builder/case-builder.js'
+import { CaseNode } from '../operation-node/case-node.js'
+import { isUndefined } from '../util/object-utils.js'
 
 export interface ExpressionBuilder<DB, TB extends keyof DB> {
   /**
@@ -150,6 +155,65 @@ export interface ExpressionBuilder<DB, TB extends keyof DB> {
     from: TE
   ): SelectQueryBuilder<From<DB, TE>, FromTables<DB, TB, TE>, {}>
 
+  /**
+   * Creates a `case` statement/operator.
+   *
+   * ### Examples
+   *
+   * Kitchen sink example with 2 flavors of `case` operator:
+   *
+   * ```ts
+   * import { sql } from 'kysely'
+   *
+   * const { title, name } = await db
+   *   .selectFrom('person')
+   *   .where('id', '=', '123')
+   *   .select((eb) => [
+   *     eb.fn.coalesce('last_name', 'first_name').as('name'),
+   *     eb
+   *       .case()
+   *       .when('gender', '=', 'male')
+   *       .then('Mr.')
+   *       .when('gender', '=', 'female')
+   *       .then(
+   *         eb
+   *           .case('martialStatus')
+   *           .when('single')
+   *           .then('Ms.')
+   *           .else('Mrs.')
+   *           .end()
+   *       )
+   *       .end()
+   *       .as('title'),
+   *   ])
+   *   .executeTakeFirstOrThrow()
+   * ```
+   *
+   * The generated SQL (PostgreSQL):
+   *
+   * ```sql
+   * select
+   *   coalesce("last_name", "first_name") as "name",
+   *   case
+   *     when "gender" = $1 then $2
+   *     when "gender" = $3 then
+   *       case "martialStatus"
+   *         when $4 then $5
+   *         else $6
+   *       end
+   *   end as "title"
+   * from "person"
+   * where "id" = $7
+   * ```
+   */
+  case(): CaseBuilder<DB, TB>
+
+  case<C extends SimpleReferenceExpression<DB, TB>>(
+    column: C
+  ): CaseBuilder<DB, TB, ExtractTypeFromReferenceExpression<DB, TB, C>>
+
+  case<O>(expression: Expression<O>): CaseBuilder<DB, TB, O>
+
   /**
    * This can be used to reference columns.
    *
@@ -505,6 +569,18 @@ export function createExpressionBuilder<DB, TB extends keyof DB>(
       })
     },
 
+    case<RE extends ReferenceExpression<DB, TB>>(
+      reference?: RE
+    ): CaseBuilder<DB, TB, ExtractTypeFromReferenceExpression<DB, TB, RE>> {
+      return new CaseBuilder({
+        node: CaseNode.create(
+          isUndefined(reference)
+            ? undefined
+            : parseReferenceExpression(reference)
+        ),
+      })
+    },
+
     ref<RE extends StringReference<DB, TB>>(
       reference: RE
     ): ExpressionWrapper<ExtractTypeFromReferenceExpression<DB, TB, RE>> {

src/index.ts

@@ -6,6 +6,7 @@ export {
   ExpressionBuilder,
   expressionBuilder,
 } from './expression/expression-builder.js'
+export * from './expression/expression-wrapper.js'
 
 export * from './query-builder/where-interface.js'
 export * from './query-builder/returning-interface.js'
@@ -22,6 +23,7 @@ export * from './query-builder/delete-result.js'
 export * from './query-builder/update-result.js'
 export * from './query-builder/on-conflict-builder.js'
 export * from './query-builder/aggregate-function-builder.js'
+export * from './query-builder/case-builder.js'
 
 export * from './raw-builder/raw-builder.js'
 export * from './raw-builder/sql.js'
@@ -102,6 +104,7 @@ export * from './operation-node/alias-node.js'
 export * from './operation-node/alter-column-node.js'
 export * from './operation-node/alter-table-node.js'
 export * from './operation-node/and-node.js'
+export * from './operation-node/case-node.js'
 export * from './operation-node/check-constraint-node.js'
 export * from './operation-node/column-definition-node.js'
 export * from './operation-node/column-node.js'
@@ -167,6 +170,7 @@ export * from './operation-node/update-query-node.js'
 export * from './operation-node/value-list-node.js'
 export * from './operation-node/value-node.js'
 export * from './operation-node/values-node.js'
+export * from './operation-node/when-node.js'
 export * from './operation-node/where-node.js'
 export * from './operation-node/with-node.js'
 export * from './operation-node/explain-node.js'

src/kysely.ts

@@ -7,7 +7,7 @@ import { QueryCreator, QueryCreatorProps } from './query-creator.js'
 import { KyselyPlugin } from './plugin/kysely-plugin.js'
 import { DefaultQueryExecutor } from './query-executor/default-query-executor.js'
 import { DatabaseIntrospector } from './dialect/database-introspector.js'
-import { freeze, isObject } from './util/object-utils.js'
+import { freeze, isObject, isUndefined } from './util/object-utils.js'
 import { RuntimeDriver } from './driver/runtime-driver.js'
 import { SingleConnectionProvider } from './driver/single-connection-provider.js'
 import {
@@ -27,6 +27,10 @@ import { QueryResult } from './driver/database-connection.js'
 import { CompiledQuery } from './query-compiler/compiled-query.js'
 import { createQueryId, QueryId } from './util/query-id.js'
 import { Compilable, isCompilable } from './util/compilable.js'
+import { CaseBuilder } from './query-builder/case-builder.js'
+import { CaseNode } from './operation-node/case-node.js'
+import { parseExpression } from './parser/expression-parser.js'
+import { Expression } from './expression/expression.js'
 import { WithSchemaPlugin } from './plugin/with-schema/with-schema-plugin.js'
 
 /**
@@ -142,6 +146,23 @@ export class Kysely<DB>
     return this.#props.dialect.createIntrospector(this.withoutPlugins())
   }
 
+  /**
+   * Creates a `case` statement/operator.
+   *
+   * See {@link ExpressionBuilder.case} for more information.
+   */
+  case(): CaseBuilder<DB, keyof DB>
+
+  case<V>(value: Expression<V>): CaseBuilder<DB, keyof DB, V>
+
+  case<V>(value?: Expression<V>): any {
+    return new CaseBuilder({
+      node: CaseNode.create(
+        isUndefined(value) ? undefined : parseExpression(value)
+      ),
+    })
+  }
+
   /**
    * Returns a {@link FunctionModule} that can be used to write type safe function
    * calls.

src/operation-node/case-node.ts

@@ -0,0 +1,59 @@
+import { freeze } from '../util/object-utils.js'
+import { OperationNode } from './operation-node.js'
+import { WhenNode } from './when-node.js'
+
+export interface CaseNode extends OperationNode {
+  readonly kind: 'CaseNode'
+  readonly value?: OperationNode
+  readonly when?: ReadonlyArray<WhenNode>
+  readonly else?: OperationNode
+  readonly isStatement?: boolean
+}
+
+/**
+ * @internal
+ */
+export const CaseNode = freeze({
+  is(node: OperationNode): node is CaseNode {
+    return node.kind === 'CaseNode'
+  },
+
+  create(value?: OperationNode): CaseNode {
+    return freeze({
+      kind: 'CaseNode',
+      value,
+    })
+  },
+
+  cloneWithWhen(caseNode: CaseNode, when: WhenNode): CaseNode {
+    return freeze({
+      ...caseNode,
+      when: freeze(caseNode.when ? [...caseNode.when, when] : [when]),
+    })
+  },
+
+  cloneWithThen(caseNode: CaseNode, then: OperationNode): CaseNode {
+    return freeze({
+      ...caseNode,
+      when: caseNode.when
+        ? freeze([
+            ...caseNode.when.slice(0, -1),
+            WhenNode.cloneWithResult(
+              caseNode.when[caseNode.when.length - 1],
+              then
+            ),
+          ])
+        : undefined,
+    })
+  },
+
+  cloneWith(
+    caseNode: CaseNode,
+    props: Partial<Pick<CaseNode, 'else' | 'isStatement'>>
+  ): CaseNode {
+    return freeze({
+      ...caseNode,
+      ...props,
+    })
+  },
+})

src/operation-node/operation-node-transformer.ts

@@ -80,6 +80,8 @@ import { BinaryOperationNode } from './binary-operation-node.js'
 import { UnaryOperationNode } from './unary-operation-node.js'
 import { UsingNode } from './using-node.js'
 import { FunctionNode } from './function-node.js'
+import { CaseNode } from './case-node.js'
+import { WhenNode } from './when-node.js'
 
 /**
  * Transforms an operation node tree into another one.
@@ -194,6 +196,8 @@ export class OperationNodeTransformer {
     UnaryOperationNode: this.transformUnaryOperation.bind(this),
     UsingNode: this.transformUsing.bind(this),
     FunctionNode: this.transformFunction.bind(this),
+    CaseNode: this.transformCase.bind(this),
+    WhenNode: this.transformWhen.bind(this),
   })
 
   transformNode<T extends OperationNode | undefined>(node: T): T {
@@ -898,6 +902,24 @@ export class OperationNodeTransformer {
     })
   }
 
+  protected transformCase(node: CaseNode): CaseNode {
+    return requireAllProps<CaseNode>({
+      kind: 'CaseNode',
+      value: this.transformNode(node.value),
+      when: this.transformNodeList(node.when),
+      else: this.transformNode(node.else),
+      isStatement: node.isStatement,
+    })
+  }
+
+  protected transformWhen(node: WhenNode): WhenNode {
+    return requireAllProps<WhenNode>({
+      kind: 'WhenNode',
+      condition: this.transformNode(node.condition),
+      result: this.transformNode(node.result),
+    })
+  }
+
   protected transformDataType(node: DataTypeNode): DataTypeNode {
     // An Object.freezed leaf node. No need to clone.
     return node

src/operation-node/operation-node-visitor.ts

@@ -82,6 +82,8 @@ import { BinaryOperationNode } from './binary-operation-node.js'
 import { UnaryOperationNode } from './unary-operation-node.js'
 import { UsingNode } from './using-node.js'
 import { FunctionNode } from './function-node.js'
+import { WhenNode } from './when-node.js'
+import { CaseNode } from './case-node.js'
 
 export abstract class OperationNodeVisitor {
   protected readonly nodeStack: OperationNode[] = []
@@ -171,6 +173,8 @@ export abstract class OperationNodeVisitor {
     UnaryOperationNode: this.visitUnaryOperation.bind(this),
     UsingNode: this.visitUsing.bind(this),
     FunctionNode: this.visitFunction.bind(this),
+    CaseNode: this.visitCase.bind(this),
+    WhenNode: this.visitWhen.bind(this),
   })
 
   protected readonly visitNode = (node: OperationNode): void => {
@@ -268,4 +272,6 @@ export abstract class OperationNodeVisitor {
   protected abstract visitUnaryOperation(node: UnaryOperationNode): void
   protected abstract visitUsing(node: UsingNode): void
   protected abstract visitFunction(node: FunctionNode): void
+  protected abstract visitCase(node: CaseNode): void
+  protected abstract visitWhen(node: WhenNode): void
 }

src/operation-node/operation-node.ts

@@ -78,6 +78,8 @@ export type OperationNodeKind =
   | 'UnaryOperationNode'
   | 'UsingNode'
   | 'FunctionNode'
+  | 'CaseNode'
+  | 'WhenNode'
 
 export interface OperationNode {
   readonly kind: OperationNodeKind

src/operation-node/when-node.ts

@@ -0,0 +1,31 @@
+import { freeze } from '../util/object-utils.js'
+import { OperationNode } from './operation-node.js'
+
+export interface WhenNode extends OperationNode {
+  readonly kind: 'WhenNode'
+  readonly condition: OperationNode
+  readonly result?: OperationNode
+}
+
+/**
+ * @internal
+ */
+export const WhenNode = freeze({
+  is(node: OperationNode): node is WhenNode {
+    return node.kind === 'WhenNode'
+  },
+
+  create(condition: OperationNode): WhenNode {
+    return freeze({
+      kind: 'WhenNode',
+      condition,
+    })
+  },
+
+  cloneWithResult(whenNode: WhenNode, result: OperationNode): WhenNode {
+    return freeze({
+      ...whenNode,
+      result,
+    })
+  },
+})