COALESCE

SELECT * FROM EMPLOYEE_PAY_TBL;
+-----------+---------------+------------+----------+-----------------+----------+---------+
| EMP_ID    | POSITION      | DATE_HIRE  | PAY_RATE | DATE_LAST_RAISE | SALARY   | BONUS   |
+-----------+---------------+------------+----------+-----------------+----------+---------+
| 213764555 | SALES MANAGER | 2004-08-14 |     NULL | 2009-08-01      | 30000.00 | 2000.00 |
| 220984332 | SHIPPER       | 2006-07-22 |    11.00 | 1999-07-01      |     NULL |    NULL |
| 311549902 | MARKETING     | 1999-05-23 |     NULL | 2009-05-01      | 40000.00 |    NULL |
| 313782439 | SALSEMAN      | 2007-06-28 |     NULL | NULL            | 20000.00 | 1000.00 |
| 442346889 | TEAM LEADER   | 2000-06-17 |    14.75 | 2009-06-01      |     NULL |    NULL |
| 443679012 | SHIPPER       | 2001-01-14 |    15.00 | 1999-01-01      |     NULL |    NULL |
+-----------+---------------+------------+----------+-----------------+----------+---------+

SELECT EMP_ID, COALESCE(BONUS, SALARY, PAY_RATE) FROM EMPLOYEE_PAY_TBL;
-- 接受一个数据集，依次检查其中每一个值，直到发现一个非NULL值。如果没有找到非NULL值，返回一个NULL值
+-----------+-----------------------------------+
| EMP_ID    | COALESCE(BONUS, SALARY, PAY_RATE) |
+-----------+-----------------------------------+
| 213764555 |                           2000.00 |
| 220984332 |                             11.00 |
| 311549902 |                          40000.00 |
| 313782439 |                           1000.00 |
| 442346889 |                             14.75 |
| 443679012 |                             15.00 |
+-----------+-----------------------------------+