Question

I have defined:
table X

ID    AGE    HEIGHT   REGISTERED
1     23     1.83     1
2     24     1.81     0
3     24     1.72     0
4     22     1.91     1
5     32     1.76     0

And I would like to perform a query to return valid values but *ONLY* for the registered ID’s. Those who are not, would get 0 in return. like this:

result

ID    AGE    HEIGHT   REGISTERED
1     23     1.83     1
2     0      0        0
3     0      0        0
4     22     1.91     1
5     0      0        0

What is the way of defining that query?

Answer 1

You can use the MySQL specific IF expression:

SELECT 
    ID,
    IF(REGISTERED, AGE, 0) AS AGE,
    IF(REGISTERED, HEIGHT, 0) AS HEIGHT,
    REGISTERED
FROM tablex

See it working online: sqlfiddle

You could also use the standard SQL CASE expression:

SELECT 
    ID,
    CASE WHEN REGISTERED = 1 THEN AGE ELSE 0 END AS AGE,
    CASE WHEN REGISTERED = 1 THEN HEIGHT ELSE 0 END AS HEIGHT,
    REGISTERED
FROM tablex

In response to your comment here’s one more that might be slightly easier if you have lots of columns:

SELECT id, AGE, HEIGHT, REGISTERED
FROM tablex
WHERE REGISTERED

UNION ALL

SELECT id, 0, 0, 0
FROM tablex
WHERE NOT REGISTERED

ORDER BY ID

See it working online: sqlfiddle

Note that the column list in this query could be shortened to just SELECT * and the query would still work. But using SELECT * in production code is not advisable for various reasons that have been covered elsewhere on the site.