Abstracted the null type check

Author: zachmu

sql/expression/case.go

@@ -45,7 +45,8 @@ func NewCase(expr sql.Expression, branches []CaseBranch, elseExpr sql.Expression
 
 // Type implements the sql.Expression interface.
 func (c *Case) Type() sql.Type {
-	curr := types.Null
+	var curr sql.Type
+	curr = types.Null
 	for _, b := range c.Branches {
 		curr = types.GeneralizeTypes(curr, b.Value.Type())
 	}

sql/expression/function/coalesce.go

@@ -58,7 +58,9 @@ func (c *Coalesce) Type() sql.Type {
 	if c.typ != nil {
 		return c.typ
 	}
-	retType := types.Null
+	
+	var retType sql.Type
+	retType = types.Null
 	for i, arg := range c.args {
 		if arg == nil {
 			continue

sql/type.go

@@ -104,6 +104,9 @@ type Type interface {
 // NullType represents the type of NULL values
 type NullType interface {
 	Type
+	
+	// IsNullType is a marker interface for types that represent NULL values.
+	IsNullType() bool
 }
 
 // DeferredType is a placeholder for prepared statements

sql/types/conversion.go

@@ -642,10 +642,10 @@ func GeneralizeTypes(a, b sql.Type) sql.Type {
 		return a
 	}
 
-	if a == Null {
+	if IsNullType(a) {
 		return b
 	}
-	if b == Null {
+	if IsNullType(b) {
 		return a
 	}
 

sql/types/null.go

@@ -34,6 +34,10 @@ var (
 
 type nullType struct{}
 
+func (t nullType) IsNullType() bool {
+	return true
+}
+
 // Compare implements Type interface. Note that while this returns 0 (equals)
 // for ordering purposes, in SQL NULL != NULL.
 func (t nullType) Compare(s context.Context, a interface{}, b interface{}) (int, error) {

sql/types/typecheck.go

@@ -106,6 +106,14 @@ func IsNumber(t sql.Type) bool {
 	}
 }
 
+func IsNullType(t sql.Type) bool {
+	nt, ok := t.(sql.NullType)
+	if !ok {
+		return false
+	}
+	return nt.IsNullType()
+}
+
 // IsSigned checks if t is a signed type.
 func IsSigned(t sql.Type) bool {
 	if svt, ok := t.(sql.SystemVariableType); ok {