Assign a case value to a column rather than an alias - sql

Question

This should be a simple one, but I have not found any solution:
The normal way is using an alias like this:
CASE WHEN ac_code='T' THEN 'time' ELSE 'purchase' END as alias
When using alias in conjunction with UNION ALL this causes problem because the alias is not treated the same way as the other columns.
Using an alias to assign the value is not working. It is still treated as alias, though it has the column name.
CASE WHEN ac_code='T' THEN 'time' ELSE 'purchase' END as ac_subject
I want to assign a value to a column based on a condition.
CASE WHEN ac_code='T' THEN ac_subject ='time' ELSE ac_subject='purchase' END
Now I get the error message
UNION types character varying and boolean cannot be matched
How can I assign a value to a column in a case statement without using an alias in the column (shared by other columns in UNION)?
Here is the whole (simplified) query:
SELECT hr_id,
CASE WHEN hr_subject='' THEN code_name ELSE hr_subject END
FROM hr
LEFT JOIN code ON code_id=hr_code
WHERE hr_job='123'
UNION ALL
SELECT po_id,
CASE WHEN po_subject='' THEN code_name ELSE po_subject END
FROM po
LEFT JOIN code ON code_id=po_code
WHERE po_job='123'
UNION ALL
SELECT ac_id,
CASE WHEN ac_code='T' THEN ac_subject='time' ELSE ac_subject='purchase' END
FROM ac
WHERE ac_job='123'

Answer 1

There is no alias in your presented query. You are confusing terms. This would be a column alias:
CASE WHEN hr_subject='' THEN code_name ELSE hr_subject END AS ac_subject
In a UNION query, the number of columns, column names and data types in the returned set are determined by the first row. All appended rows have to match the row type. Column names in appended rows (including aliases) are just noise and ignored. Maybe useful for documentation, nothing else.
The = operator does not assign anything in a SELECT query. It's the equality operator that returns a boolean value. TRUE if both operands are equal, etc. This returns a boolean value: ac_subject='time' Hence your error message:
UNION types character varying and boolean cannot be matched
The only way to "assign" a value to a particular output column in this query is to include it at the right position in the SELECT list.
The information in the question is incomplete, but I suspect you are also confusing the empty string ('') with the NULL value. A distinction that you need to understand before doing anything else with relational databases. Maybe start here. In this case you would rather use COALESCE to provide a default for NULL values:
SELECT hr_id, COALESCE(hr_subject, code_name) AS ac_subject
FROM hr
LEFT JOIN code ON code_id=hr_code
WHERE hr_job = '123'
UNION ALL
SELECT po_id, COALESCE(po_subject, code_name)
FROM po
LEFT JOIN code ON code_id=po_code
WHERE po_job = '123'
UNION ALL
SELECT ac_id, CASE WHEN ac_code = 'T' THEN 'time'::varchar ELSE 'purchase' END
FROM ac
WHERE ac_job = '123'
Just an educated guess, assuming type varchar. You should have added table qualification to column names to clarify their origin. Or table definitions to clarify everything.

Answer 2

The CASE expression is supposed to return a value, e.g. 'time'.
Your value is another expression subject ='time' which is a boolean (true or false).
Is this on purpose? Does the other query you glue with UNION have a boolean in that place, too? Probably not, and this is what the DBMS complains about.

Answer 3

I found the problem.
CASE WHEN hr_subject=’’ THEN code_name ELSE hr_subject END
The columns code_name and hr_subject was different length. This caused the unpredictable result. I think that aliases can work now.
Thank you for your support.