mysql case statement fails to create a new column - mysql

Question

Sorry about asking the basics. I am trying to make a new column named new_column as in the code below while keeping the old_column as well. When I run the below query, I do get the results printed out fine on the console, but failed to insert the results to the new_column.
I tried both ways, first creating a column named new_column (empty) and then run the query, and secondly just running the query as below. Both didn't return new_column inserted with integer values. What am I missing?
SELECT old_column CASE
WHEN 'A' THEN 1
WHEN 'B' THEN 2
WHEN 'C' THEN 3
ELSE 0
END AS new_column
FROM table_name

Answer 1

Your syntax is wrong, and doesn't produce anything except an error message as you've written it here.
If I understand your question correctly, this should produce the output you want:
SELECT
old_column,
CASE old_column
WHEN 'A' THEN 1
WHEN 'B' THEN 2
WHEN 'C' THEN 3
ELSE 0
END AS new_column
FROM
table_name
A SELECT does exactly what it says it does - it selects the data, but doesn't actually alter it's content.
To permanently add a new column and populate it, you'll need to first ALTER TABLE to add the new column, and then UPDATE that new column's content.
ALTER TABLE table_name ADD COLUMN newcolumn integer;
UPDATE
table_name
SET newcolumn = CASE oldcolumn
WHEN 'A' THEN 1
WHEN 'B' THEN 2
WHEN 'C' THEN 3
ELSE 0
END;
For future reference: If you want help with your SQL syntax, include the actual code you've tried that isn't working, instead of inventing something as you go or re-typing. With SQL-related questions, it's almost always a good idea to post some sample data and the output you'd like to obtain from that data, along with your actual SQL code trying to produce that output. It makes it much easier to help you when we can understand what you're trying to do and have samples to use.