count id from 1 table where his id is not in second table

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how to count all user id of the first table that don't have refer id in the second table?

Table 1:
+-----+------------+---------------------+
| id  | first_name | created_at          |
+-----+------------+---------------------+
| 522 | dddd       | 2013-04-11 18:44:07 |
| 523 | cccc       | 2013-03-01 06:19:12 |
| 524 | ffff       | 2013-03-06 21:07:06 |
| 525 | rrrr       | 2013-03-06 22:05:08 |
| 590 | tttt       | 2013-03-16 16:39:59 |
| 584 | yyyy       | 2013-03-22 14:46:28 |
 
Table 2:
+-----+---------------------+---------------------+
| id  | referred_by_user_id | created_at          |
+-----+---------------------+---------------------+
| 298 |                 522 | 2013-02-22 22:19:06 |
| 299 |                 584 | 2013-03-22 18:27:49 |
| 300 |                 584 | 2013-03-22 18:48:23 |
| 301 |                 584 | 2013-03-22 19:01:40 |
| 302 |                 584 | 2013-03-22 19:05:24 |
| 303 |                 584 | 2013-03-22 19:13:18 |
| 304 |                 590 | 2013-04-10 23:24:17 |
| 305 |                 522 | 2013-04-11 18:44:07 |

Posted 22-Apr-13 9:24am

Yafa Su

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OriginalGriff · Accepted Answer · 2013-04-22T09:33:00

Solution 1

Try:

SQL

SELECT COUNT(*) FROM Table1 t1 
LEFT JOIN Table2 t2 ON t1.id=t2.referred_by_user_id 
WHERE t2.referred_by_user_id IS NULL

Posted 22-Apr-13 9:33am

OriginalGriff

Comments

Yafa Su 22-Apr-13 15:49pm

Thank it works, but if i replace 'is null' to '= 4' it's not working you know why?

OriginalGriff 22-Apr-13 16:14pm

:laugh: Yes!
It has to do with what a left join is: and W3Schools.com can explain it better than I can.

StianSandberg 22-Apr-13 17:21pm

Perfect. 5'd