SQL Server update same column using different conversion without overwriting

nxagd54h 于 2023-03-11 发布在 SQL Server

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I have a table DATES that looks like this:
| Id | TimeStamp | TimeStamp_UTC |
| ------------ | ------------ | ------------ |
| 1 | 2021-08-03 12:10:30 | 2021-08-03 12:10:30.000 |
| 2 | 2021-09-13 12:21:44 | NULL |
| 3 | 11/23/2021 1:30:56.511 PM | NULL |
| 4 | 11/23/2021 1:37:27.476 PM | NULL |

The column TimeStamp is of type nvarchar and TimeStamp_UTC of type datetime .

I want to convert the data from TimeStamp into the column TimeStamp_UTC by using just one query.

That implies using two CONVERT functions, one for the first two dates and a slightly different one for the last two.

The SQL statements to convert both types are shown here:

For the first two:

UPDATE DATES
SET [TimeStamp_UTC_JM] = (SELECT CONVERT(datetime, [TimeStamp], 20) 
WHERE LEN([TimeStamp]) IN (18, 19))

For the other two dates:

UPDATE DATES 
SET [TimeStamp_UTC_JM] = (SELECT CONVERT(datetime, [TimeStamp], 21) 
WHERE LEN([TimeStamp]) BETWEEN 23 AND 26)

Individually both updates work, but when running the second query the converted values of the first update disappear, so I would like to perform the update in just one step, without overwriting.

sql-server

来源：https://stackoverflow.com/questions/75693670/sql-server-update-same-column-using-different-conversion-without-overwriting

2条答案

按热度按时间

bvn4nwqk1#

Just run these two UPDATE statements after each other - check to ensure you're not overwriting any existing values in Timestamp_UTC in your WHERE clause:

-- update the first style of date/time formats
UPDATE dbo.Dates
SET [TimeStamp_UTC] = CONVERT(DATETIME2(3), [TimeStamp], 120) 
WHERE LEN([TimeStamp]) IN (18, 19)
  AND TimeStamp_UTC IS NULL;

-- update the second style of date/time formats
UPDATE dbo.Dates
SET [TimeStamp_UTC] = CONVERT(DATETIME2(3), [TimeStamp], 101) 
WHERE LEN([TimeStamp]) BETWEEN 23 AND 26
  AND TimeStamp_UTC IS NULL;

I had to also change the styles used for conversion - since you have 4-digit years, those have to be styles in the 100er range - and the second style you had doesn't exist in 4-digit years - but style = 101 seems to work just fine.

I also used DATETIME2(3) as datatype, since this is recommended since the days of SQL Server 2008 - it has a better precision, uses less memory to store its values, and has a larger range of supported dates - basically nothing but benefits over using the old DATETIME datatype. I'd strongly recommend defining any new columns to store date and time with the DATETIME2(n) datatype and phase out DATETIME

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slwdgvem2#

Another solution that worked in my case was, as I said, using just one CONVERT function for both datetime cases:

UPDATE DATES
SET TimeStamp_UTC = (SELECT ISNULL(TimeStamp_UTC, CONVERT(datetime, [TimeStamp], 21)))

赞(0）回复(0）举报 2023-03-11

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SQL Server update same column using different conversion without overwriting

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