feat(spanner/spansql): add support for nullif expressions (#6423)

Co-authored-by: rahul2393 <irahul@google.com>

spanner/spansql/parser.go

@@ -3044,6 +3044,9 @@ func (p *parser) parseLit() (Expr, *parseError) {
 	case tok.caseEqual("IFNULL"):
 		p.back()
 		return p.parseIfNullExpr()
+	case tok.caseEqual("NULLIF"):
+		p.back()
+		return p.parseNullIfExpr()
 	}
 
 	// Handle typed literals.
@@ -3210,6 +3213,30 @@ func (p *parser) parseIfNullExpr() (IfNull, *parseError) {
 	return IfNull{Expr: expr, NullResult: nullResult}, nil
 }
 
+func (p *parser) parseNullIfExpr() (NullIf, *parseError) {
+	if err := p.expect("NULLIF", "("); err != nil {
+		return NullIf{}, err
+	}
+
+	expr, err := p.parseExpr()
+	if err != nil {
+		return NullIf{}, err
+	}
+	if err := p.expect(","); err != nil {
+		return NullIf{}, err
+	}
+
+	exprToMatch, err := p.parseExpr()
+	if err != nil {
+		return NullIf{}, err
+	}
+	if err := p.expect(")"); err != nil {
+		return NullIf{}, err
+	}
+
+	return NullIf{Expr: expr, ExprToMatch: exprToMatch}, nil
+}
+
 func (p *parser) parseArrayLit() (Array, *parseError) {
 	// ARRAY keyword is optional.
 	// TODO: If it is present, consume any <T> after it.

spanner/spansql/parser_test.go

@@ -446,6 +446,12 @@ func TestParseExpr(t *testing.T) {
 				NullResult: True,
 			},
 		},
+		{`NULLIF("a", "b")`,
+			NullIf{
+				Expr:        StringLiteral("a"),
+				ExprToMatch: StringLiteral("b"),
+			},
+		},
 
 		// String literal:
 		// Accept double quote and single quote.

spanner/spansql/sql.go

@@ -753,6 +753,15 @@ func (in IfNull) addSQL(sb *strings.Builder) {
 	sb.WriteString(")")
 }
 
+func (ni NullIf) SQL() string { return buildSQL(ni) }
+func (ni NullIf) addSQL(sb *strings.Builder) {
+	sb.WriteString("NULLIF(")
+	ni.Expr.addSQL(sb)
+	sb.WriteString(", ")
+	ni.ExprToMatch.addSQL(sb)
+	sb.WriteString(")")
+}
+
 func (b BoolLiteral) SQL() string { return buildSQL(b) }
 func (b BoolLiteral) addSQL(sb *strings.Builder) {
 	if b {

spanner/spansql/sql_test.go

@@ -685,6 +685,20 @@ func TestSQL(t *testing.T) {
 			`SELECT IFNULL(10, 0)`,
 			reparseQuery,
 		},
+		{
+			Query{
+				Select: Select{
+					List: []Expr{
+						NullIf{
+							Expr:        IntegerLiteral(10),
+							ExprToMatch: IntegerLiteral(0),
+						},
+					},
+				},
+			},
+			`SELECT NULLIF(10, 0)`,
+			reparseQuery,
+		},
 	}
 	for _, test := range tests {
 		sql := test.data.SQL()

spanner/spansql/types.go

@@ -760,6 +760,14 @@ type IfNull struct {
 func (IfNull) isBoolExpr() {} // possibly bool
 func (IfNull) isExpr()     {}
 
+type NullIf struct {
+	Expr        Expr
+	ExprToMatch Expr
+}
+
+func (NullIf) isBoolExpr() {} // possibly bool
+func (NullIf) isExpr()     {}
+
 type BoolLiteral bool
 
 const (