mysql简单查询

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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 tableCREATE 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*12FROM 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 empWHERE 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 deptnoFROM emp;/**   可以使用DISTINCT清除重复记录*/SELECT DISTINCT deptnoFROM 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,jobFROM empWHERE deptno=20;

8. where限定查询结果

/**   使用where可以限定返回的记录*/  SELECT *FROM empWHERE 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 empWHERE ename='SMITH';SELECT *FROM empWHERE hiredate='1980-12-17';

10. 比较运算符

SELECT ename,sal,comm,jobFROM empWHERE 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,jobFROM empWHERE 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,jobFROM empWHERE mgr IN(7902,7566,7788);SELECT ename,sal,comm,jobFROM empWHERE 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,jobFROM empWHERE ename LIKE '%A%';SELECT ename,sal,comm,jobFROM empWHERE ename LIKE '_L%';

13. AND OR

# 两个条件都满足SELECT ename,sal,comm,jobFROM empWHERE sal>1000 AND job='SALESMAN';# 两个条件满足一个SELECT ename,sal,comm,jobFROM empWHERE sal>1000 OR job='SALESMAN';

14. 对结果集进行排序

#order bySELECT *FROM empORDER BY sal DESC;  #从高到低SELECT *FROM empORDER BY sal ASC;  #从低到高

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