added GeneralizeTypes function

Author: angelamayxie

sql/expression/function/if.go

@@ -95,17 +95,7 @@ func (f *If) Eval(ctx *sql.Context, row sql.Row) (interface{}, error) {
 
 // Type implements the Expression interface.
 func (f *If) Type() sql.Type {
-	// if either type is string type, this should be a string type, regardless need to promote
-	typ1 := f.ifTrue.Type()
-	typ2 := f.ifFalse.Type()
-	if types.IsText(typ1) || types.IsText(typ2) {
-		return types.Text
-	}
-
-	if typ1 == types.Null {
-		return typ2.Promote()
-	}
-	return typ1.Promote()
+	return types.GeneralizeTypes(f.ifTrue.Type(), f.ifFalse.Type())
 }
 
 // CollationCoercibility implements the interface sql.CollationCoercible.

sql/expression/function/ifnull.go

@@ -69,13 +69,7 @@ func (f *IfNull) Eval(ctx *sql.Context, row sql.Row) (interface{}, error) {
 
 // Type implements the Expression interface.
 func (f *IfNull) Type() sql.Type {
-	if types.IsNull(f.LeftChild) {
-		if types.IsNull(f.RightChild) {
-			return types.Null
-		}
-		return f.RightChild.Type()
-	}
-	return f.LeftChild.Type()
+	return types.GeneralizeTypes(f.LeftChild.Type(), f.RightChild.Type())
 }
 
 // CollationCoercibility implements the interface sql.CollationCoercible.

sql/types/conversion.go

@@ -554,3 +554,24 @@ func TypesEqual(a, b sql.Type) bool {
 		return a.Equals(b)
 	}
 }
+
+// GeneralizeTypes returns the more "general" of two types as defined by
+// https://dev.mysql.com/doc/refman/8.4/en/flow-control-functions.html#function_if and
+// https://dev.mysql.com/doc/refman/8.4/en/flow-control-functions.html#function_ifnull
+// TODO: Currently returns the most general type. Update to match MySQL (pick the more general of the two given types)
+func GeneralizeTypes(a, b sql.Type) sql.Type {
+	if IsText(a) || IsText(b) {
+		// TODO: handle case-sensitive strings
+		return Text
+	}
+
+	if IsFloat(a) || IsFloat(b) {
+		return Float64
+	}
+
+	if a == Null {
+		return b.Promote()
+	}
+
+	return a.Promote()
+}