types/datum.go: casting zero to enum should not change value

executor/executor_test.go

@@ -3314,6 +3314,16 @@ func (s *testSuite) TestEmptyEnum(c *C) {
 	tk.MustQuery("select * from t").Check(testkit.Rows("", ""))
 	tk.MustExec("insert into t values (null)")
 	tk.MustQuery("select * from t").Check(testkit.Rows("", "", "<nil>"))
+
+	// Test https://github.com/pingcap/tidb/issues/29525.
+	tk.MustExec("drop table if exists t;")
+	tk.MustExec("create table t (id int auto_increment primary key, c1 enum('a', '', 'c'));")
+	tk.MustExec("insert into t(c1) values (0);")
+	tk.MustQuery("select id, c1+0, c1 from t;").Check(testkit.Rows("1 0 "))
+	tk.MustExec("alter table t change c1 c1 enum('a', '') not null;")
+	tk.MustQuery("select id, c1+0, c1 from t;").Check(testkit.Rows("1 0 "))
+	tk.MustExec("insert into t(c1) values (0);")
+	tk.MustQuery("select id, c1+0, c1 from t;").Check(testkit.Rows("1 0 ", "2 0 "))
 }
 
 // TestIssue4024 This tests https://github.com/pingcap/tidb/issues/4024

expression/integration_test.go

@@ -9536,6 +9536,11 @@ func (s *testIntegrationSuite) TestEnumPushDown(c *C) {
 	tk.MustExec("insert into tdm values (1, 'a');")
 	tk.MustExec("update tdm set c12 = 2 where id = 1;")
 	tk.MustQuery("select * from tdm").Check(testkit.Rows("1 b"))
+	tk.MustExec("set @@sql_mode = '';")
+	tk.MustExec("update tdm set c12 = 0 where id = 1;")
+	tk.MustQuery("select c12+0 from tdm").Check(testkit.Rows("0"))
+	tk.MustExec("update tdm set c12 = '0' where id = 1;")
+	tk.MustQuery("select c12+0 from tdm").Check(testkit.Rows("0"))
 }
 
 func (s *testIntegrationSuite) TestJiraSetInnoDBDefaultRowFormat(c *C) {

expression/scalar_function.go

@@ -364,6 +364,14 @@ func (sf *ScalarFunction) Eval(row chunk.Row) (d types.Datum, err error) {
 		str, isNull, err = sf.EvalString(sf.GetCtx(), row)
 		if !isNull && err == nil && tp.Tp == mysql.TypeEnum {
 			res, err = types.ParseEnum(tp.Elems, str, tp.Collate)
+			if ctx := sf.GetCtx(); ctx != nil {
+				if sc := ctx.GetSessionVars().StmtCtx; sc != nil {
+					if sc.TruncateAsWarning {
+						ctx.GetSessionVars().StmtCtx.AppendWarning(err)
+						err = nil
+					}
+				}
+			}
 		} else {
 			res = str
 		}

types/datum.go

@@ -1499,19 +1499,24 @@ func (d *Datum) convertToMysqlEnum(sc *stmtctx.StatementContext, target *FieldTy
 	case KindString, KindBytes, KindBinaryLiteral:
 		e, err = ParseEnum(target.Elems, d.GetString(), target.Collate)
 	case KindMysqlEnum:
-		e, err = ParseEnum(target.Elems, d.GetMysqlEnum().Name, target.Collate)
+		if d.i == 0 {
+			// MySQL enum zero value has an empty string name(Enum{Name: '', Value: 0}). It is
+			// different from the normal enum string value(Enum{Name: '', Value: n}, n > 0).
+			e = Enum{}
+		} else {
+			e, err = ParseEnum(target.Elems, d.GetMysqlEnum().Name, target.Collate)
+		}
 	case KindMysqlSet:
 		e, err = ParseEnum(target.Elems, d.GetMysqlSet().Name, target.Collate)
 	default:
 		var uintDatum Datum
 		uintDatum, err = d.convertToUint(sc, target)
 		if err == nil {
 			e, err = ParseEnumValue(target.Elems, uintDatum.GetUint64())
+		} else {
+			err = errors.Wrap(ErrTruncated, "convert to MySQL enum failed: "+err.Error())
 		}
 	}
-	if err != nil {
-		err = errors.Wrap(ErrTruncated, "convert to MySQL enum failed: "+err.Error())
-	}
 	ret.SetMysqlEnum(e, target.Collate)
 	return ret, err
 }

types/enum.go

@@ -15,6 +15,7 @@
 package types
 
 import (
+	"fmt"
 	"strconv"
 
 	"github.com/pingcap/errors"
@@ -55,8 +56,8 @@ func ParseEnum(elems []string, name string, collation string) (Enum, error) {
 	if num, err := strconv.ParseUint(name, 0, 64); err == nil {
 		return ParseEnumValue(elems, num)
 	}
-
-	return Enum{}, errors.Errorf("item %s is not in enum %v", name, elems)
+	errMsg := fmt.Sprintf("convert to MySQL enum failed: item %s is not in enum %v", name, elems)
+	return Enum{}, errors.Wrap(ErrTruncated, errMsg)
 }
 
 // ParseEnumName creates a Enum with item name.
@@ -67,14 +68,15 @@ func ParseEnumName(elems []string, name string, collation string) (Enum, error)
 			return Enum{Name: n, Value: uint64(i) + 1}, nil
 		}
 	}
-
-	return Enum{}, errors.Errorf("item %s is not in enum %v", name, elems)
+	errMsg := fmt.Sprintf("convert to MySQL enum failed: item %s is not in enum %v", name, elems)
+	return Enum{}, errors.Wrap(ErrTruncated, errMsg)
 }
 
 // ParseEnumValue creates a Enum with special number.
 func ParseEnumValue(elems []string, number uint64) (Enum, error) {
 	if number == 0 || number > uint64(len(elems)) {
-		return Enum{}, errors.Errorf("number %d overflow enum boundary [1, %d]", number, len(elems))
+		errMsg := fmt.Sprintf("convert to MySQL enum failed: number %d overflow enum boundary [1, %d]", number, len(elems))
+		return Enum{}, errors.Wrap(ErrTruncated, errMsg)
 	}
 
 	return Enum{Name: elems[number-1], Value: number}, nil