SELECT查询命令的使用

--1、查询数据

--查询所有的数据

SELECT * FROM EMP T;

--查询指定列的数据(最后一个指定列与FROM不需要用逗号隔开)

SELECT T.EMPNO,T.JOB,T.SAL FROM EMP T;

--查询去重复的数据(关键字DISTINC是去除相同列中重复的字段)

--去重只能在同一个列中进行

SELECT DISTINCT T.MGR FROM EMP T;

SELECT DISTINCT T.SAL FROM EMP T;

--2、查询数据中的使用+-*/表达式，包含数字和日期；

--数字

SELECT  T.EMPNO, T.MGR , T.EMPNO+T.MGR ADDNUM FROM EMP T;

SELECT  T.EMPNO, T.MGR , T.EMPNO-T.MGR ADDNUM FROM EMP T;

SELECT  T.EMPNO, T.MGR , T.EMPNO*T.MGR ADDNUM FROM EMP T;

SELECT  T.EMPNO, T.MGR , T.EMPNO/T.MGR ADDNUM FROM EMP T;

--日期(只能参与加减运算，都是在日上面进行叠加或叠减到月和年)

SELECT T.ENAME,T.HIREDATE,T.HIREDATE+6 FROM EMP T;

SELECT T.ENAME,T.HIREDATE,T.HIREDATE+6 FROM EMP T;

--指定日期的格式（使用TO_CHAR()方法）

SELECT T.HIREDATE,TO_CHAR(T.HIREDATE+6,'yyyy-mm-dd') NOWDATE FROM EMP T;

--3、查询数据中处理空值(使用NVL（）函数)

SELECT NVL(T.MGR,0),NVL(T.SAL,0),NVL(T.MGR,0)+NVL(T.SAL,0) NOWADD  FROM EMP T;

--4、使用排序查询，包含升序和降序(排序子句ORDER BY)

--排序规则使用列名，别名，表达式或序号

--升序（ACS关键字 此时为自动）和降序（DESC关键字）

--列名

SELECT T.EMPNO,T.MGR FROM EMP T ORDER BY T.EMPNO ASC;

SELECT T.EMPNO,T.MGR FROM EMP T ORDER BY T.EMPNO ;

SELECT T.EMPNO,T.MGR FROM EMP T ORDER BY T.EMPNO DESC;

--别名

SELECT T.SAL GONGZI FROM EMP T ORDER BY GONGZI ASC;

SELECT T.COMM JIANGJIN FROM EMP T ORDER BY JIANGJIN ASC; 

SELECT T.SAL GONGZI FROM EMP T ORDER BY GONGZI DESC;

SELECT T.DEPTNO JIANG FROM EMP T ORDER BY JIANG DESC; 

SELECT T.COMM JI FROM EMP T ORDER BY JI DESC; 

--表达式

SELECT T.EMPNO,T.MGR,T.EMPNO+T.MGR HE FROM EMP T ORDER BY HE ASC;

SELECT T.EMPNO,T.MGR,T.EMPNO+T.MGR HE FROM EMP T ORDER BY HE DESC; 

--序号

SELECT T.EMPNO,T.MGR,T.EMPNO+T.MGR HE FROM EMP T ORDER BY 1;

SELECT T.EMPNO,T.MGR,T.EMPNO+T.MGR HE FROM EMP T ORDER BY 1 DESC;

--5、where条件查询：

--使用 条件=   >   >=   <   <=  !=   <>   ^=

--字符串的精确查找以首字母为对象

SELECT * FROM EMP T WHERE T.EMPNO=7521;

SELECT * FROM EMP T WHERE T.ENAME='ALLEN';

SELECT * FROM EMP T WHERE T.SAL>1000;

SELECT * FROM EMP T WHERE T.JOB>'A';

SELECT * FROM EMP T WHERE T.SAL>=1000;

SELECT * FROM EMP T WHERE T.JOB>='A';

SELECT * FROM EMP T WHERE T.SAL<1000;

SELECT * FROM EMP T WHERE T.JOB<'A'
4000
;

SELECT * FROM EMP T WHERE T.SAL<=1000;

SELECT * FROM EMP T WHERE T.JOB<='A';

--三个都是不等于

SELECT * FROM EMP T WHERE T.SAL!=1000;

SELECT * FROM EMP T WHERE T.JOB<>'A';

SELECT * FROM EMP T WHERE T.SAL^=1000;

--BETWEEN ... AND... 

SELECT * FROM EMP T WHERE T.SAL BETWEEN 1000 AND 2000;

SELECT * FROM EMP T WHERE T.JOB BETWEEN 'A' AND 'J';

--IN(list)(使用的个数有限)

--此处是精确查询，类似于java中的switch(){case1:}

--字符串必须要用单引号括起来；

SELECT * FROM EMP T WHERE T.SAL IN(800,950,3000);

SELECT * FROM EMP T WHERE T.JOB IN('CLERK','SALESMAN');

--LIKE

SELECT * FROM EMP T WHERE T.ENAME LIKE 'S%'; --以S开头的

SELECT * FROM EMP T WHERE T.ENAME LIKE '%S';

SELECT * FROM EMP T WHERE T.JOB LIKE '__S%';

SELECT * FROM EMP T WHERE T.ENAME LIKE '%S%';

--IS NULL

SELECT * FROM EMP T WHERE T.SAL IS NULL;

--NOT BETWEEN ... AND... 

SELECT * FROM EMP T WHERE T.SAL BETWEEN 1000 AND 2000;

SELECT * FROM EMP T WHERE T.JOB BETWEEN 'A' AND 'J';

--NOT IN(list)

SELECT * FROM EMP T WHERE T.SAL NOT IN(800,950,3000);

SELECT * FROM EMP T WHERE T.JOB NOT IN('CLERK','SALESMAN');

-- NOT LIKE

SELECT * FROM EMP T WHERE T.ENAME  NOT LIKE 'S%'; 

SELECT * FROM EMP T WHERE T.ENAME NOT LIKE '%S';

SELECT * FROM EMP T WHERE T.JOB NOT LIKE '__S%';

SELECT * FROM EMP T WHERE T.ENAME NOT LIKE '%S%';

-- NOT IS NULL

SELECT * FROM EMP T WHERE T.SAL IS NOT NULL;