示例

CREATE TABLE employee (
employee_id INT,
first_name VARCHAR(50),
last_name VARCHAR(50),
manager_id INT,
salary NUMERIC
);
INSERT INTO employee VALUES(111, 'DEL', 'FA BEN', 1, 1500);
INSERT INTO employee VALUES(112, 'AXEL', 'BELL', 1, 1000);
INSERT INTO employee VALUES(113, 'CRIS',  'RACHAR', 1, 1000);

• 查询部分列：

SELECT first_name, last_name, salary FROM employee;
+------------+-----------+--------+
| FIRST_NAME | LAST_NAME | SALARY |
| DEL        | FA BEN    |   1500 |
| AXEL       | BELL      |   1000 |
+------------+-----------+--------+

• 查询所有列：

• 对列进行数学运算：

SELECT salary+100 FROM employee;
+------------+
| SALARY+100 |
+------------+
|       1600 |
|       1100 |
|       1100 |
+------------+

• 给列取别名：

SELECT salary*12 年薪 FROM employee;
+--------+
| 年薪   |
+--------+
|  18000 |
+--------+

• 字符串拼接：

• 数据去重：

SELECT DISTINCT MANAGER_ID FROM employee;
+------------+
| MANAGER_ID |
+------------+
|          1 |
+------------+

• CASE WHEN 语句：

SELECT salary, CASE WHEN salary >= 1000 then '高薪' WHEN salary >= 800 THEN '一般' 
ELSE '继续努力' END AS typeFROM employee;
+--------+--------+
| SALARY | TYPE   |
+--------+--------+
|   1500 | 高薪   |
|   1000 | 高薪   |
|   1000 | 高薪   |