Support subquery in case then. Closes #811

src/expression/expression-builder.ts

@@ -353,7 +353,9 @@ export interface ExpressionBuilder<DB, TB extends keyof DB> {
     column: C
   ): CaseBuilder<DB, TB, ExtractTypeFromReferenceExpression<DB, TB, C>>
 
-  case<O>(expression: Expression<O>): CaseBuilder<DB, TB, O>
+  case<E extends Expression<any>>(
+    expression: E
+  ): CaseBuilder<DB, TB, ExtractTypeFromValueExpression<E>>
 
   /**
    * This method can be used to reference columns within the query's context. For

src/query-builder/case-builder.ts

@@ -10,6 +10,7 @@ import {
   parseValueBinaryOperationOrExpression,
 } from '../parser/binary-operation-parser.js'
 import {
+  ExtractTypeFromValueExpression,
   isSafeImmediateValue,
   parseSafeImmediateValue,
   parseValueExpression,
@@ -72,7 +73,9 @@ export class CaseThenBuilder<DB, TB extends keyof DB, W, O> {
    * A `then` call can be followed by {@link Whenable.when}, {@link CaseWhenBuilder.else},
    * {@link CaseWhenBuilder.end} or {@link CaseWhenBuilder.endCase} call.
    */
-  then<O2>(expression: Expression<O2>): CaseWhenBuilder<DB, TB, W, O | O2>
+  then<E extends Expression<any>>(
+    expression: E
+  ): CaseWhenBuilder<DB, TB, W, O | ExtractTypeFromValueExpression<E>>
 
   then<V>(value: V): CaseWhenBuilder<DB, TB, W, O | V>
 

test/typings/test-d/case.test-d.ts

@@ -94,6 +94,15 @@ async function testCase(eb: ExpressionBuilder<Database, 'person'>) {
       .end()
   )
 
+  // subquery
+  expectType<ExpressionWrapper<Database, 'person', string | null>>(
+    eb
+      .case()
+      .when('gender', '=', 'male')
+      .then(eb.selectFrom('person').select('first_name'))
+      .end()
+  )
+
   // errors
 
   expectError(eb.case().when('no_such_column', '=', 'male').then('Mr.').end())