Date and time types

https://docs.microsoft.com/en-us/sql/t-sql/data-types/date-and-time-types

date (Transact-SQL)
datetime (Transact-SQL)
datetime2 (Transact-SQL)
datetimeoffset (Transact-SQL)
smalldatetime (Transact-SQL)
time (Transact-SQL)

https://docs.microsoft.com/en-us/sql/t-sql/functions/date-and-time-data-types-and-functions-transact-sql

https://stackoverflow.com/questions/1334143/datetime2-vs-datetime-in-sql-server

CREATE TABLE TestTime
    (
        TempTime TIME ,
        TempDate DATE ,
        TempSMALLDATETIME SMALLDATETIME ,
        TempDateTime DATETIME ,
        TempDateTime2 DATETIME2 ,
        TempDATETIMEOFFSET DATETIMEOFFSET
    );

SELECT *
FROM   dbo.TestTime;

DECLARE @TempTime TIME;
DECLARE @TempDate DATE;
DECLARE @TempSMALLDATETIME SMALLDATETIME;
DECLARE @TempDateTime DATETIME;
DECLARE @TempDateTime2 DATETIME2;
DECLARE @TempDATETIMEOFFSET DATETIMEOFFSET;

SELECT @TempTime;
SELECT @TempDate;
SELECT @TempSMALLDATETIME;
SELECT @TempDateTime;
SELECT @TempDateTime2;
SELECT @TempDATETIMEOFFSET;

TIMEFROMPARTS ( hour, minute, seconds, fractions, precision )

fractions表示不到一秒的时间的数值。【比如1毫秒，10毫秒】

precision决定了fractions的时间单位。因为Time的精度是100纳秒，所以precision的取值范围是[1,7]。

然后fractions和precision组合起来，是不允许超过1秒的。

如果precision为3，那么fraction的单位是10^-3秒，即1毫秒。

如果precision为6，那么fraction的单位是10^-6秒，即1微秒。

如果precision为9，那么fraction的单位是10^-9秒，即1纳秒。

如果precision为2，那么fraction的单位是10^-2秒，即10毫秒。

SELECT TIMEFROMPARTS(23, 59, 59, POWER(10, 7)-1, 7) --23:59:59.9999999

DATETIMEFROMPARTS ( year, month, day, hour, minute, seconds, milliseconds )

DateTime的精度是0.00333秒，即3.33毫秒

SELECT DATETIMEFROMPARTS(2017, 12, 31, 23, 59, 59, 991) ,--2017-12-31 23:59:59.990
       DATETIMEFROMPARTS(2017, 12, 31, 23, 59, 59, 992) ,--2017-12-31 23:59:59.993
       DATETIMEFROMPARTS(2017, 12, 31, 23, 59, 59, 993) ,--2017-12-31 23:59:59.993
       DATETIMEFROMPARTS(2017, 12, 31, 23, 59, 59, 994) ,--2017-12-31 23:59:59.993
       DATETIMEFROMPARTS(2017, 12, 31, 23, 59, 59, 995) ,--2017-12-31 23:59:59.997
       DATETIMEFROMPARTS(2017, 12, 31, 23, 59, 59, 996) ,--2017-12-31 23:59:59.997
       DATETIMEFROMPARTS(2017, 12, 31, 23, 59, 59, 997) ,--2017-12-31 23:59:59.997
       DATETIMEFROMPARTS(2017, 12, 31, 23, 59, 59, 998) ,--2017-12-31 23:59:59.997
       DATETIMEFROMPARTS(2017, 12, 31, 23, 59, 59, 999);,--2018-01-01 00:00:00.000