add datetime comparison test

Author: angelamayxie

enginetest/queries/queries.go

@@ -4281,6 +4281,10 @@ SELECT * FROM cte WHERE  d = 2;`,
 		ExpectedWarningsCount: 1,
 		Expected:              []sql.Row{{time.Date(2020, time.January, 1, 12, 34, 56, 123456000, time.UTC)}},
 	},
+	{
+		Query:    "select cast('2020-01-01 12:34:56.123456' as datetime(6)) > cast('2020-01-01 12:34:56' as datetime)",
+		Expected: []sql.Row{{true}},
+	},
 	{
 		Query: `SELECT * FROM (SELECT * FROM (SELECT * FROM (SELECT * FROM othertable) othertable_one) othertable_two) othertable_three WHERE s2 = 'first'`,
 		Expected: []sql.Row{

sql/types/datetime.go

@@ -104,8 +104,6 @@ var (
 	Date = MustCreateDatetimeType(sqltypes.Date, 0)
 	// Datetime is a date and a time with default precision (no fractional seconds).
 	Datetime = MustCreateDatetimeType(sqltypes.Datetime, 0)
-	// DatetimeDefaultPrecision is a date and a time without a specified precision
-	DatetimeDefaultPrecision = MustCreateDatetimeType(sqltypes.Datetime, 0)
 	// DatetimeMaxPrecision is a date and a time with maximum precision
 	DatetimeMaxPrecision = MustCreateDatetimeType(sqltypes.Datetime, MaxDatetimePrecision)
 	// Timestamp is a UNIX timestamp with default precision (no fractional seconds).