查询常数：
SELECT 'MySQL',employee_id,last_name
FROM employees;
mysql> SELECT 'MySQL',employee_id,last_name -> FROM employees; +-------+-------------+-------------+ | MySQL | employee_id | last_name | +-------+-------------+-------------+ | MySQL | 100 | King | | MySQL | 101 | Kochhar | | MySQL | 102 | De Haan | | MySQL | 103 | Hunold | | MySQL | 104 | Ernst | | MySQL | 105 | Austin | | MySQL | 106 | Pataballa |