Postgres: cast, money and NULL values

Question

In my select I am using this in order to convert an integer to a money form.

CAST(mytable.discount AS money) AS Discount

But I cannot figure out how to avoid the 'NULL' output if the join fails (for good cause) to bring the optional value.

I've done this to avoid NULLS in the past:

COALESCE(mytable.voucher,'----') AS Voucher

But I cannot figure out how to combined CAST and COALESCE for the same field. I just want my discount NULL fields to be '----'

That's tricky, as the "mytable.discount AS money" converts NULLS to $0, which is what it outputs. '-----' never comes to play. — JasonGenX
– JasonGenX, Commented Feb 24, 2015 at 22:32
"I just want my discount NULL fields to be '----'" - you can't. '---' is a character value, discount is a number. You can't have different datatypes for the same column. You would need to convert all numbers to a string as value (e.g. using to_char()) — user330315
– user330315, Commented Feb 24, 2015 at 22:40

zerkms · Accepted Answer · 2015-02-24 22:40:37Z

1

That's tricky, as the "mytable.discount AS money" converts NULLS to $0

It's actually not what happens, but an implicit cast which happens after that.

An expression must have a particular type. In this case it's money. So you see $0.00 as a result of my proposed expression because it's ---- that is converted to money, not NULL.

As a solution you may explicitly convert the inner expression to text like:

SELECT COALESCE(CAST('1' as money)::text, '--');

or

SELECT COALESCE(CAST(null as money)::text, '--');

SQLFiddle demo: http://sqlfiddle.com/#!12/d41d8/2866

answered Feb 24, 2015 at 22:40

zerkms

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