SELECT last_name AS name
FROM employees;
SELECT last_name AS name, department_id "部门id"
FROM employees;
SELECT last_name AS name, department_id "部门id", salary * 12 "annual sal"
FROM employees;
SELECT DISTINCT department_id
FROM employees;
#错误的:没有去重的情况
SELECT department_id
FROM employees;
#仅仅是没有报错,但是没有实际意义。
SELECT DISTINCT department_id,salary
FROM employees;
这里有两点需要注意:
SELECT salary, DISTINCT department_id FROM employees
会报错。null
值,运算的结果都为null
null
不等同于0,'','null'
SELECT * FROM employees;
SELECT employee_id,salary "月工资",salary * (1 + commission_pct) * 12 "年工资",commission_pct
FROM employees;
IFNULL
IFNULL(a,b) :如果参数a不是null,就加入计算,如果是null就拿参数b来计算SELECT employee_id,salary "月工资",salary * (1 + IFNULL(commission_pct,0)) * 12 "年工资",commission_pct
FROM employees;SELECT * FROM order
> 1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'order' at line 1
mysql> SELECT * FROM `ORDER`;
+----------+------------+
| order_id | order_name |
+----------+------------+
| 1 | shkstart |
| 2 | tomcat |
| 3 | dubbo |
+----------+------------+
3 rows in set (0.00 sec)
mysql> SELECT * FROM `order`;
+----------+------------+
| order_id | order_name |
+----------+------------+
| 1 | shkstart |
| 2 | tomcat |
| 3 | dubbo |
+----------+------------+
3 rows in set (0.00 sec)
employees
数据表中的员工姓名进行查询,同时增加一列字段corporation
,这个字段固定值为“兮动人
”,可以这样写:SELECT '兮动人' as corporation, last_name
FROM employees;DESCRIBE employees; #显示了表中字段的详细信息
或
DESC employees;
其中,各个字段的含义分别解释如下:
SELECT *
FROM employees
#过滤条件,声明在FROM结构的后面
WHERE department_id = 90;
last_name
为'King
'的员工信息SELECT *
FROM EMPLOYEES
WHERE LAST_NAME = 'King';
King
' 也是可以任意大小写的ANNUAL SALARY
SELECT employee_id, last_name, salary * 12 "ANNUAL SALARY"
FROM employees
employees
表中去除重复的job_id
以后的数据SELECT DISTINCT job_id
FROM employees
12000
的员工姓名和工资SELECT last_name salary
FROM employees
WHERE salary > 12000;
176
的员工的姓名和部门号SELECT last_name department_id
FROM employees
WHERE employee_id = 176
departments
的结构DESC departments;
原创声明:本文系作者授权腾讯云开发者社区发表,未经许可,不得转载。
如有侵权,请联系 cloudcommunity@tencent.com 删除。
原创声明:本文系作者授权腾讯云开发者社区发表,未经许可,不得转载。
如有侵权,请联系 cloudcommunity@tencent.com 删除。