fix default value for non null enum columns

enginetest/queries/script_queries.go

@@ -7900,6 +7900,31 @@ where
 			},
 		},
 	},
+	{
+		Name:    "special case for not null default enum",
+		Dialect: "mysql",
+		SetUpScript: []string{
+			"create table t (i int primary key, e enum('abc', 'def', 'ghi') not null);",
+		},
+		Assertions: []ScriptTestAssertion{
+			{
+				Query: "insert into t(i) values (1)",
+				Expected: []sql.Row{
+					{types.NewOkResult(1)},
+				},
+			},
+			{
+				Query:       "insert into t values (2, null)",
+				ExpectedErr: sql.ErrInsertIntoNonNullableProvidedNull,
+			},
+			{
+				Query: "select * from t;",
+				Expected: []sql.Row{
+					{1, "abc"},
+				},
+			},
+		},
+	},
 	{
 		Name:    "not expression optimization",
 		Dialect: "mysql",

sql/rowexec/insert.go

@@ -80,6 +80,18 @@ func (i *insertIter) Next(ctx *sql.Context) (returnRow sql.Row, returnErr error)
 		row = row[len(row)-len(i.schema):]
 	}
 
+	// This is a special case in MySQL.
+	// When there's an enum column with a NOT NULL constraint, the DEFAULT value is the first entry.
+	for idx, col := range i.schema {
+		if idx >= len(i.insertExprs) {
+			break
+		}
+		_, isColDefVal := i.insertExprs[idx].(*sql.ColumnDefaultValue)
+		if row[idx] == nil && types.IsEnum(col.Type) && isColDefVal {
+			row[idx] = 1
+		}
+	}
+
 	err = i.validateNullability(ctx, i.schema, row)
 	if err != nil {
 		return nil, i.ignoreOrClose(ctx, row, err)