[CALCITE-6104] Aggregate function that references outer column should be evaluated in outer query

core/src/main/java/org/apache/calcite/runtime/CalciteResource.java

@@ -924,6 +924,9 @@ ExInst<CalciteException> illegalArgumentForTableFunctionCall(String a0,
   @BaseMessage("Extended columns not allowed under the current SQL conformance level")
   ExInst<SqlValidatorException> extendNotAllowed();
 
+  @BaseMessage("Aggregate function referencing outer column is not allowed under the current SQL conformance level")
+  ExInst<SqlValidatorException> correlatedAggregateNotAllowed();
+
   @BaseMessage("Rolled up column ''{0}'' is not allowed in {1}")
   ExInst<SqlValidatorException> rolledUpNotAllowed(String column, String context);
 

core/src/main/java/org/apache/calcite/sql/validate/SqlAbstractConformance.java

@@ -176,4 +176,8 @@ public abstract class SqlAbstractConformance implements SqlConformance {
   @Override public boolean isDistinctOnAllowed() {
     return SqlConformanceEnum.DEFAULT.isDistinctOnAllowed();
   }
+
+  @Override public boolean isCorrelatedAggregateAllowed() {
+    return SqlConformanceEnum.DEFAULT.isCorrelatedAggregateAllowed();
+  }
 }

core/src/main/java/org/apache/calcite/sql/validate/SqlConformance.java

@@ -691,4 +691,20 @@ default boolean isColonFieldAccessAllowed() {
    * false otherwise.
    */
   boolean isDistinctOnAllowed();
+
+  /**
+   * Whether an aggregate function inside a scalar sub-query is allowed to
+   * reference columns from an outer query.
+   *
+   * <p>This is not allowed by the SQL standard, but is supported by some
+   * databases, including SQLite, DuckDB and SQL Server.
+   *
+   * <p>Among the built-in conformance levels, true in
+   * {@link SqlConformanceEnum#BABEL},
+   * {@link SqlConformanceEnum#LENIENT};
+   * false otherwise.
+   */
+  default boolean isCorrelatedAggregateAllowed() {
+    return false;
+  }
 }

core/src/main/java/org/apache/calcite/sql/validate/SqlConformanceEnum.java

@@ -542,4 +542,14 @@ public enum SqlConformanceEnum implements SqlConformance {
       return false;
     }
   }
+
+  @Override public boolean isCorrelatedAggregateAllowed() {
+    switch (this) {
+    case BABEL:
+    case LENIENT:
+      return true;
+    default:
+      return false;
+    }
+  }
 }

core/src/main/java/org/apache/calcite/sql/validate/SqlDelegatingConformance.java

@@ -181,4 +181,8 @@ protected SqlDelegatingConformance(SqlConformance delegate) {
   @Override public boolean isDistinctOnAllowed() {
     return delegate.isDistinctOnAllowed();
   }
+
+  @Override public boolean isCorrelatedAggregateAllowed() {
+    return delegate.isCorrelatedAggregateAllowed();
+  }
 }