Question

I have a temporary table in a PostgreSQL function and I want to insert a new VARCHAR column. It should have a value that depends on another column of the table, named “amount”.

When the amount is positive I would like the value of the column row to be credit and when the value is negative the column should be debit.

I have one more request: I want to round the value of amount column in 2 decimal digits

Answer 1

You want ALTER TABLE ... ADD COLUMN followed by an UPDATE.

I initially said ALTER TABLE ... ADD COLUMN ... USING but that was wrong on two counts. ADD COLUMN takes a DEFAULT not USING – and You can’t do it in one pass because neither a DEFAULT expression nor a USING expression may not refer to other columns.

So you must do:

 ALTER TABLE tablename ADD COLUMN colname varchar;
 UPDATE tablename SET colname = ( CASE WHEN othercol < 0 THEN 'Credit' ELSE 'Debit' END );

Think carefully about whether zero should be ‘Debit’ or ‘Credit’ and adjust the CASE accordingly.

For rounding, use round(amount,2). There isn’t enough detail in your question for me to be sure how; probably by UPDATEing the temp table with UPDATE thetable SET amount = round(amount,2) but without the context it’s hard to know if that’s right. That statement irreversibly throws information away so it should only be used on a copy of the data.