14.MySQL中的DISTINCT(去除重复记录)

 1 CREATE DATABASE mahaiwuji;
 2 USE mahaiwuji;
 3 CREATE TABLE emp
 4 (
 5     empno INT PRIMARY KEY,
 6     ename VARCHAR(10),
 7     sal INT,
 8     deptno INT
 9 );
10 INSERT INTO emp VALUES (1,‘smith‘,300,10);
11 INSERT INTO emp VALUES (2,‘allen‘,NULL,10);
12 INSERT INTO emp VALUES (3,‘ward‘,NULL,20);
13 INSERT INTO emp VALUES (4,‘jones‘,NULL,10);
14 INSERT INTO emp VALUES (5,‘martin‘,NULL,30);
15 INSERT INTO emp VALUES (6,‘blake‘,200,20);
16 INSERT INTO emp VALUES (7,‘clark‘,0,30);

1 SELECT DISTINCT deptno FROM emp;

1 -- DISTINCT可以去掉多余的NULL，或者说如果有多个NULL，只保留一个
2 -- 0不是NULL
3 SELECT DISTINCT sal FROM emp;

1 SELECT DISTINCT sal,deptno FROM emp;