CODEWITHSUNDEEP
sqlmysql
Swati
Swati

Reputation: 52757

Updating multiple rows with a value calculated from another column

I have a table with a row that looks like this:

(2009123148498429, '...', '...')

The first part, id, is a timestamp followed by a random number. (needed to work with other parts in the system) The data already exists in the table.

I want to create a column, timestamp, and extract just the date (20091231) and update all the rows with the timestamp.

  1. How can I do this for all the rows with SQL? (i.e. update them all with some sort of a function?)
  2. What kind of default value should I assign the column to make sure that future inserts correctly extract the date?

UPDATE - Please read the comments by bobince in the first answered question by Jonathan Sampson on how we got to the answer. This is the final query that worked:

UPDATE table SET rdate=substring(convert(rid,char(20)),1,8);

The problem was that I was using substring as substring( str, 0, 8 ) whereas it should be substring( str, 1, 8 ). I guess we're all used to 0 as the beginning position! More info here on substring

Related to: multiple updates in mysql

Upvotes: 3

Views: 3775

Answers (4)

Chris Hynes
Chris Hynes

Reputation: 10239

UPDATE tbl
SET
   newDateField = SUBSTRING(CAST(sourceCol AS varchar), 0, 8)

Upvotes: 0

lc.
lc.

Reputation: 116498

Use a sub-select in your update (untested, and I've been using Firebird for too long, so someone check me here).

UPDATE MyTable AS TUpdate
SET MyNewField = (SELECT SUBSTRING(TSub.MyDateColumn,0,8) 
                  FROM MyTable AS TSub 
                  WHERE TSub.MyID = TUpdate.MyID);

As for future inserts correctly extracting the date, you're going to have to create a trigger on insert to extract the date for that record.

Upvotes: 1

Suroot
Suroot

Reputation: 4423

Need to use a subselect.

UPDATE someTable set timestamp = (SELECT SUBSTRING(colData, 0, 8) FROM someOriginalTable);

EDIT lc got me by a few seconds!

Upvotes: 0

Sampson
Sampson

Reputation: 268364

SELECT SUBSTRING(colDate,0,8) as 'date' 
FROM someTable

Or am I mistaken?

UPDATE someTable
SET newDateField = SUBSTRING(colDate,0,8)

Would likely work too. Untested.

Upvotes: 2

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