fix issue:pingcap#11102

Unexcepted result in `SELECT ... CASE WHEN ... ELSE NULL...`

expression/constant_fold.go

@@ -23,11 +23,19 @@ import (
 // specialFoldHandler stores functions for special UDF to constant fold
 var specialFoldHandler = map[string]func(*ScalarFunction) (Expression, bool){}
 
+// specialNullRejectCheck stores function names for special UDF to skip constant fold.
+var specialNullRejectCheck = map[string]struct{}{}
+
 func init() {
 	specialFoldHandler = map[string]func(*ScalarFunction) (Expression, bool){
 		ast.If:     ifFoldHandler,
 		ast.Ifnull: ifNullFoldHandler,
 	}
+
+	specialNullRejectCheck = map[string]struct{}{
+		ast.NullEQ: struct{}{},
+		ast.Case:	struct{}{},
+	}
 }
 
 // FoldConstant does constant folding optimization on an expression excluding deferred ones.
@@ -104,7 +112,8 @@ func foldConstant(expr Expression) (Expression, bool) {
 			isDeferredConst = isDeferredConst || isDeferred
 		}
 		if !allConstArg {
-			if !hasNullArg || !sc.InNullRejectCheck || x.FuncName.L == ast.NullEQ {
+			_, ok :=  specialNullRejectCheck[x.FuncName.L]
+			if !hasNullArg || !sc.InNullRejectCheck || ok {
 				return expr, isDeferredConst
 			}
 			constArgs := make([]Expression, len(args))

session/session_test.go

@@ -2722,3 +2722,18 @@ func (s *testSessionSuite) TestLoadClientInteractive(c *C) {
 	tk.Se.GetSessionVars().ClientCapability = tk.Se.GetSessionVars().ClientCapability | mysql.ClientInteractive
 	tk.MustQuery("select @@wait_timeout").Check(testkit.Rows("28800"))
 }
+
+func (s *testSessionSuite) TestFuncCaseWithLeftJoin(c *C)  {
+	tk := testkit.NewTestKitWithInit(c, s.store)
+
+	tk.MustExec("create table kankan1(id int, name text)")
+	tk.MustExec("insert into kankan1 values(1, 'a')")
+	tk.MustExec("insert into kankan1 values(2, 'a')")
+
+	tk.MustExec("create table kankan2(id int, h1 text)")
+	tk.MustExec("insert into kankan2 values(2, 'z')")
+
+	tk.MustQuery( "select * from (select t1.id, t2.h1, case when t1.name='b' then 'case2' when t1.name='a' then " +
+		"'case1' else null end as flag from kankan1 t1 left join kankan2 t2 on t1.id = t2.id) t3 where t3.flag='case1' " +
+		"order by t3.id").Check(testkit.Rows("1 <nil> case1", "2 z case1"))
+}