aswam1975
asked on
how to write an update trigger
I would like to write an update trigger which does the following.
When a column called counter is updated in table1 , the trigger updates the counter value in table2.
eg:
table 1 has columns id, counter
table2 columns id, counter.
Now if the id in table 1 does not exist in table2, then when table1 's counter is updated that id and counter are inserted into table2.
I have never triggers before and would appreciate any help
Thanks in advance
When a column called counter is updated in table1 , the trigger updates the counter value in table2.
eg:
table 1 has columns id, counter
table2 columns id, counter.
Now if the id in table 1 does not exist in table2, then when table1 's counter is updated that id and counter are inserted into table2.
I have never triggers before and would appreciate any help
Thanks in advance
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It returns a constant that doesn't take up much space -- the purpose of that query is to just find any rows that meet the where criteria -- i.e., to see if there are any rows in table2 that have an "id" column whose value matches the new record's id column value. Depending on the server involved, it might handle this select by actually creating a table with the result set and then checking to see if that table has any rows. If that's the strategy that the server uses, then it makes sense to construct the smallest table possible. I'm not sure how modern implementations of PostgreSQL handle such a query -- it's possible that they just check for the existence of a row and don't actually build the temporary table containing the result set. However, such a trick is common and should result in the fastest query on pretty much any SQL engine.
ASKER
thanks again!
ASKER
Can you let me know what the term "select 1" does?