fix issue:pingcap#11102

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

expression/constant_fold.go

@@ -15,6 +15,7 @@ package expression
 
 import (
 	"github.com/pingcap/parser/ast"
+	"github.com/pingcap/tidb/types"
 	"github.com/pingcap/tidb/util/chunk"
 	"github.com/pingcap/tidb/util/logutil"
 	"go.uber.org/zap"
@@ -23,18 +24,11 @@ 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: {},
-		ast.Case:   {},
+		ast.Case:   caseWhenHandler,
 	}
 }
 
@@ -86,6 +80,51 @@ func ifNullFoldHandler(expr *ScalarFunction) (Expression, bool) {
 	return expr, isDeferredConst
 }
 
+func caseWhenHandler(expr *ScalarFunction) (Expression, bool) {
+	args, l := expr.GetArgs(), len(expr.GetArgs())
+	firstCondition := true
+	isDeferredConst := false
+	for i := 0; i < l-1; i += 2 {
+		foldedArg, isDeferred := foldConstant(args[i])
+		expr.GetArgs()[i] = foldedArg
+		isDeferredConst = isDeferredConst || isDeferred
+		if _, isConst := foldedArg.(*Constant); isConst {
+			condition, isNull, err := args[i].EvalInt(expr.GetCtx(), chunk.Row{})
+			if err != nil {
+				return expr, false
+			}
+			if firstCondition && condition != 0 && !isNull {
+				return retProcess(args[i+1], expr.GetType())
+			}
+		} else {
+			firstCondition = false
+		}
+		foldedArg1, isDeferred1 := foldConstant(args[i+1])
+		expr.GetArgs()[i+1] = foldedArg1
+		isDeferredConst = isDeferredConst || isDeferred1
+	}
+
+	if l%2 == 1 && firstCondition {
+		return retProcess(args[l-1], expr.GetType())
+	} else if l%2 == 1 {
+		foldedArg, isDeferred := foldConstant(args[l-1])
+		expr.GetArgs()[l-1] = foldedArg
+		isDeferredConst = isDeferredConst || isDeferred
+	}
+
+	return expr, isDeferredConst
+}
+
+func retProcess(expr Expression, retType *types.FieldType) (Expression, bool) {
+	foldedExpr, b := foldConstant(expr)
+	if fc, isConst := foldedExpr.(*Constant); isConst {
+		fc.RetType = retType
+		return fc, b
+	}
+
+	return foldedExpr, b
+}
+
 func foldConstant(expr Expression) (Expression, bool) {
 	switch x := expr.(type) {
 	case *ScalarFunction:
@@ -112,8 +151,7 @@ func foldConstant(expr Expression) (Expression, bool) {
 			isDeferredConst = isDeferredConst || isDeferred
 		}
 		if !allConstArg {
-			_, ok := specialNullRejectCheck[x.FuncName.L]
-			if !hasNullArg || !sc.InNullRejectCheck || ok {
+			if !hasNullArg || !sc.InNullRejectCheck || x.FuncName.L == ast.NullEQ {
 				return expr, isDeferredConst
 			}
 			constArgs := make([]Expression, len(args))

session/session_test.go

@@ -2733,7 +2733,5 @@ func (s *testSessionSuite) TestFuncCaseWithLeftJoin(c *C) {
 	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"))
+	tk.MustQuery("select t1.id from kankan1 t1 left join kankan2 t2 on t1.id = t2.id where (case  when t1.name='b' then 'case2' when t1.name='a' then 'case1' else NULL end) = 'case1' order by t1.id").Check(testkit.Rows("1", "2"))
 }