


How to Order By an Alias in PostgreSQL and Avoid the 'Column Not Exist' Error?
How to Order By an Alias in PostgreSQL: Resolving the "Column Not Exist" Error
In PostgreSQL, when working with aliases, you may encounter an error while ordering the results. For instance, consider the following query:
SELECT title, (stock_one + stock_two) AS global_stock FROM product ORDER BY global_stock = 0, title;
Executing this query in PostgreSQL 8.1.23 will likely produce the error: Query failed: ERROR: column "global_stock" does not exist. This error occurs because the alias "global_stock" is not recognized as an existing column in the "product" table.
To resolve this issue, you have a few options:
Option 1: Order By Position
PostgreSQL allows ordering by the position of the column instead of its name. For example, you can write:
select title, ( stock_one + stock_two ) as global_stock from product order by 2, 1
This query orders the results first by the second column (which is "global_stock"), and then by the first column (which is "title").
Option 2: Wrap in a Subquery
Another method involves wrapping your original query in a subquery and using the CASE statement to handle the ordering:
SELECT * from ( select title, ( stock_one + stock_two ) as global_stock from product ) x order by (case when global_stock = 0 then 1 else 0 end) desc, title
This query first creates a subquery that calculates the "global_stock" value. The subquery is then wrapped in another SELECT statement that uses the CASE statement to order the results based on the availability of items (0 for available, 1 for unavailable).
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