Question

How can I check if a provided value is a number/ integer or letters?

For instance, this is my working SQL,

SELECT 
    p.*

FROM page AS p

WHERE p.title = 'home'

AND IF(CONVERT('7', SIGNED INTEGER) IS NOT NULL, p.parent_id != p.page_id, p.parent_id = p.page_id)

‘7‘ is variable, sometime it is a number, sometimes it is ‘self‘.

So when it is a number, then p.parent_id != p.page_id

Else p.parent_id = p.page_id

But I can get it right as my SQL query returns always p.parent_id != p.page_id.

Answer 1

You can use a REGEXP.

AND CASE
    WHEN ('7' REGEXP '^[0-9]+$') THEN
      p.parent_id != p.page_id
    ELSE p.parent_id = p.page_id
END

The reason your CONVERT() fails to work as expected is that a non-numeric string will always be cast as 0, rather than NULL.

mysql> SELECT CONVERT('7a', SIGNED INTEGER);
+-------------------------------+
| CONVERT('7a', SIGNED INTEGER) |
+-------------------------------+
|                             7 |
+-------------------------------+

mysql> SELECT CONVERT('self', SIGNED INTEGER);
+---------------------------------+
| CONVERT('self', SIGNED INTEGER) |
+---------------------------------+
|                               0 | <-- non-numeric strings always cast to 0
+---------------------------------+

Your method may work if you compare against 0 instead of NULL, but I don’t have a good way to test. However, this fails if the value actually is 0, wherein you’ll get the wrong comparison back.

AND IF(CONVERT('7', SIGNED INTEGER) <> 0, p.parent_id != p.page_id, p.parent_id = p.page_id)