where in subquery

exposed/src/main/kotlin/org/jetbrains/exposed/sql/Op.kt

@@ -88,6 +88,12 @@ class InListOrNotInListOp<T>(val expr: ExpressionWithColumnType<T>, val list: It
     }
 }
 
+class InSubQueryOp<T>(val expr: Expression<T>, val query: Query): Op<Boolean>() {
+    override fun toSQL(queryBuilder: QueryBuilder): String = buildString {
+        append("${expr.toSQL(queryBuilder)} IN (${query.prepareSQL(queryBuilder)})")
+    }
+}
+
 class QueryParameter<T>(val value: T, val sqlType: IColumnType) : Expression<T>() {
     override fun toSQL(queryBuilder: QueryBuilder): String = queryBuilder.registerArgument(sqlType, value)
 }

exposed/src/main/kotlin/org/jetbrains/exposed/sql/SQLExpressionBuilder.kt

@@ -170,6 +170,8 @@ object SqlExpressionBuilder {
 
     infix fun<T> ExpressionWithColumnType<T>.notInList(list: Iterable<T>): Op<Boolean> = InListOrNotInListOp(this, list, isInList = false)
 
+    infix fun<T> ExpressionWithColumnType<T>.inSubQuery(query: Query): Op<Boolean> = InSubQueryOp(this, query)
+
     @Suppress("UNCHECKED_CAST")
     fun<T, S: T?> ExpressionWithColumnType<S>.asLiteral(value: T) = when (value) {
         is Boolean -> booleanLiteral(value)

exposed/src/test/kotlin/org/jetbrains/exposed/sql/tests/shared/DMLTests.kt

@@ -788,6 +788,14 @@ class DMLTests : DatabaseTestsBase() {
         }
     }
 
+    @Test
+    fun testInSubQuery01() {
+        withCitiesAndUsers { cities, users, userData ->
+            val r = cities.select { cities.id inSubQuery cities.slice(cities.id).select { cities.id eq 2 } }
+            assertEquals(1, r.count())
+        }
+    }
+
     @Test
     fun testInsertSelect01() {
         withCitiesAndUsers(exclude = listOf(TestDB.ORACLE)) { cities, users, userData ->