本文整理自网络,侵删。
目录
- 1.示例表内容
- 2.简单查询
- 3.给列起别名
- 4.列运算
- 5.条件查询
1.示例表内容
dept表:
+--------+------------+----------+ | DEPTNO | DNAME | LOC | +--------+------------+----------+ | 10 | ACCOUNTING | NEW YORK | | 20 | RESEARCH | DALLAS | | 30 | SALES | CHICAGO | | 40 | OPERATIONS | BOSTON | +--------+------------+----------+
emp表:
+-------+--------+-----------+------+------------+---------+---------+--------+ | EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO | +-------+--------+-----------+------+------------+---------+---------+--------+ | 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 | | 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 | | 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 | | 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 | | 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 | | 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 | | 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 | | 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 | | 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 | | 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 | | 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 | | 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 | | 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 | | 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 | +-------+--------+-----------+------+------------+---------+---------+--------+
salgrade表:
+-------+-------+-------+ | GRADE | LOSAL | HISAL | +-------+-------+-------+ | 1 | 700 | 1200 | | 2 | 1201 | 1400 | | 3 | 1401 | 2000 | | 4 | 2001 | 3000 | | 5 | 3001 | 9999 | +-------+-------+-------+
2.简单查询
MySQL 数据库使用SQL SELECT语句来查询数据。
例如我们要查询一个表的全部信息,可以这样做:
当然,这种方式进行查询的效率较低,我们更推荐您使用多列查询的方式:
mysql> select * from dept; +--------+------------+----------+ | DEPTNO | DNAME | LOC | +--------+------------+----------+ | 10 | ACCOUNTING | NEW YORK | | 20 | RESEARCH | DALLAS | | 30 | SALES | CHICAGO | | 40 | OPERATIONS | BOSTON | +--------+------------+----------+ 4 rows in set (0.00 sec)
那么,如果想查询单个列的信息呢?
可以指定列名进行查询:
mysql> select DNAME from dept; +------------+ | DNAME | +------------+ | ACCOUNTING | | RESEARCH | | SALES | | OPERATIONS | +------------+ 4 rows in set (0.00 sec)
查询多个列的信息,可以这样写:
mysql> select DNAME , LOC from dept; +------------+----------+ | DNAME | LOC | +------------+----------+ | ACCOUNTING | NEW YORK | | RESEARCH | DALLAS | | SALES | CHICAGO | | OPERATIONS | BOSTON | +------------+----------+ 4 rows in set (0.00 sec)
3.给列起别名
在查询的过程中,我们还可以选择给列起一个别名:
mysql> select DNAME as NAME from dept; +------------+ | NAME | +------------+ | ACCOUNTING | | RESEARCH | | SALES | | OPERATIONS | +------------+ 4 rows in set (0.00 sec)
也可以不加 as 关键字:
mysql> select DNAME NAME from dept; +------------+ | NAME | +------------+ | ACCOUNTING | | RESEARCH | | SALES | | OPERATIONS | +------------+ 4 rows in set (0.00 sec)
在所有的数据库中,字符串统一使用单引号,这是一个标准
4.列运算
在查询的时候,我们也可以直接进行列运算操作:
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