mysql简单查询

我们使用oracle自带的测试表进行查询学习，先使用建表语句创建数据表

dept表：

CREATE TABLE DEPT
(
  deptno INT NOT NULL PRIMARY KEY,
  dname  VARCHAR(14),
  loc    VARCHAR(13)
);

emp表：

-- Create table
CREATE TABLE EMP
(
  empno    INT NOT NULL PRIMARY KEY,
  ename    VARCHAR(10),
  job      VARCHAR(9),
  mgr      INT,
  hiredate DATE,
  sal      DOUBLE,
  comm    DOUBLE,
  deptno   INT,
  CONSTRAINT FOREIGN KEY tb_emp_fk(deptno) REFERENCES DEPT(deptno)
);

插入数据：

INSERT INTO DEPT (deptno, dname, loc)
VALUES (10, 'ACCOUNTING', 'NEW YORK');
INSERT INTO DEPT (deptno, dname, loc)
VALUES (20, 'RESEARCH', 'DALLAS');
INSERT INTO DEPT (deptno, dname, loc)
VALUES (30, 'SALES', 'CHICAGO');
INSERT INTO DEPT (deptno, dname, loc)
VALUES (40, 'OPERATIONS', 'BOSTON');

INSERT INTO EMP (empno, ename, job, mgr, hiredate, sal, comm, deptno)
VALUES (7369, 'SMITH', 'CLERK', 7902,'1980-12-17', 899, NULL, 20);
INSERT INTO EMP (empno, ename, job, mgr, hiredate, sal, comm, deptno)
VALUES (7499, 'ALLEN', 'SALESMAN', 7698,'1981-02-20', 1633, 300, 30);
INSERT INTO EMP (empno, ename, job, mgr, hiredate, sal, comm, deptno)
VALUES (7521, 'WARD', 'SALESMAN', 7698,'1981-02-22', 1250, 500, 30);
INSERT INTO EMP (empno, ename, job, mgr, hiredate, sal, comm, deptno)
VALUES (7566, 'JONES', 'MANAGER', 7839,'1981-04-02', 2975, NULL, 20);
INSERT INTO EMP (empno, ename, job, mgr, hiredate, sal, comm, deptno)
VALUES (7654, 'MARTIN', 'SALESMAN', 7698,'1981-09-28', 1250, 1400, 30);
INSERT INTO EMP (empno, ename, job, mgr, hiredate, sal, comm, deptno)
VALUES (7698, 'BLAKE', 'MANAGER', 7839,'1981-05-01', 2850, NULL, 30);
INSERT INTO EMP (empno, ename, job, mgr, hiredate, sal, comm, deptno)
VALUES (7782, 'CLARK', 'MANAGER', 7839,'1981-06-09', 2450, NULL, 10);
INSERT INTO EMP (empno, ename, job, mgr, hiredate, sal, comm, deptno)
VALUES (7788, 'SCOTT', 'ANALYST', 7566, '1987-04-19', 3000, NULL, 20);
INSERT INTO EMP (empno, ename, job, mgr, hiredate, sal, comm, deptno)
VALUES (7839, 'KING', 'PRESIDENT', NULL,'1981-11-17', 5000, NULL, 10);
INSERT INTO EMP (empno, ename, job, mgr, hiredate, sal, comm, deptno)
VALUES (7844, 'TURNER', 'SALESMAN', 7698, '1981-09-08', 1500, 0, 30);
INSERT INTO EMP (empno, ename, job, mgr, hiredate, sal, comm, deptno)
VALUES (7876, 'ADAMS', 'CLERK', 7788,'1987-05-23', 1100, NULL, 20);
INSERT INTO EMP (empno, ename, job, mgr, hiredate, sal, comm, deptno)
VALUES (7900, 'JAMES', 'CLERK', 7698,'1981-12-03', 950, NULL, 30);
INSERT INTO EMP (empno, ename, job, mgr, hiredate, sal, comm, deptno)
VALUES (7902, 'FORD', 'ANALYST', 7566,'1981-12-03', 3000, NULL, 20);
INSERT INTO EMP (empno, ename, job, mgr, hiredate, sal, comm, deptno)
VALUES (7934, 'MILLER', 'CLERK', 7782,'1982-01-23', 1300, NULL, 10);

下面，我们进行查询操作：

1. 简单的select查询

# 最简单的select语句 select 字段，字段 从 表中
SELECT deptno, dname,loc FROM dept;
SELECT * FROM dept;   # *表示所有的列
SELECT deptno FROM dept;     # 查询某一个字段的数据

SELECT * FROM emp;

2. 查询过程中的算数表达式

# 算数表达式
# 查询员工、月薪、年薪
SELECT ename,sal,sal*12
FROM emp;

3. 算数表达式的优先级

/**
   乘法和除法的优先级高于加减法
   同级运算从左到右
   表达式中括号最优先
*/

SELECT ename,sal,sal*(12+100)
FROM emp;

4.  空值 null

/** 
   空值是指不可使用、未分配的值
   空值不等于零或者空格
   任何类型都可以支持空值
   包括空值的任何算数表达式都等于空
   字符串和null进行连接运算，得到的也是null
*/

SELECT *
FROM emp
WHERE comm IS NULL;

5. 查询时为列设置别名

/**
  更改列的别名
*/

SELECT ename '名字',sal '月薪',sal*12 '年薪'
FROM emp;

6. 删除重复记录

/**
   缺省情况下查询显示所有行，包括重复记录
*/
SELECT deptno
FROM emp;

/**
   可以使用DISTINCT清除重复记录
*/
SELECT DISTINCT deptno
FROM emp;

7. DISTINCT的作用范围是后面所有字段的组合

/**
   DISTINCT的作用范围是后面所有字段的组合
*/
SELECT DISTINCT deptno,job
FROM emp
WHERE deptno=20;

8. where限定查询结果

/**
   使用where可以限定返回的记录
*/  
SELECT *
FROM emp
WHERE deptno=20;

9. 字符串和日期要用单引号引起来

/**
   字符串和日期要用单引号引起来
*/  
SELECT *
FROM emp
WHERE ename='SMITH';

SELECT *
FROM emp
WHERE hiredate='1980-12-17';

10. 比较运算符

SELECT ename,sal,comm,job
FROM emp
WHERE sal<=1500;

11. BETWEEN、 IN

/**
   使用BETWEEN运算符查询某一段数据(包含最小值和最大值)
*/  
SELECT ename,sal,comm,job
FROM emp
WHERE sal BETWEEN 1500 AND 3000;



/**
   使用IN运算符获得匹配列表值的记录
*/  
SELECT ename,sal,comm,job
FROM emp
WHERE mgr IN(7902,7566,7788);

SELECT ename,sal,comm,job
FROM emp
WHERE mgr NOT IN(7902,7566,7788);

12. LIKE进行模糊查询

/**
   使用LIKE运算符进行模糊查询
   查询条件可包含中文或数字
   (%) 可表示0或者多个字符
   (_)可表示一个字符
*/  

SELECT ename,sal,comm,job
FROM emp
WHERE ename LIKE '%A%';

SELECT ename,sal,comm,job
FROM emp
WHERE ename LIKE '_L%';

13. AND OR

# 两个条件都满足
SELECT ename,sal,comm,job
FROM emp
WHERE sal>1000 AND job='SALESMAN';


# 两个条件满足一个
SELECT ename,sal,comm,job
FROM emp
WHERE sal>1000 OR job='SALESMAN';

14. 对结果集进行排序

#order by
SELECT *
FROM emp
ORDER BY sal DESC;  #从高到低


SELECT *
FROM emp
ORDER BY sal ASC;  #从低到高