1、查看表结构
mysql> show create table emp\G
*************************** 1. row ***************************
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Table: empCreate Table: CREATE TABLE `emp` ( `empno` int(4) NOT NULL DEFAULT '0',
`ENAME` varchar(10) DEFAULT NULL,
`JOB` varchar(9) DEFAULT NULL,
`MGR` int(4) DEFAULT NULL,
`HIRE` date DEFAULT NULL,
`SAL` int(7) DEFAULT NULL,
`COMM` int(7) DEFAULT NULL,
`deptno` int(2) DEFAULT NULL,
PRIMARY KEY (`empno`),
KEY `deptno` (`deptno`),
CONSTRAINT `emp_ibfk_1` FOREIGN KEY (`deptno`) REFERENCES `dept` (`deptNO`)
) ENGINE=InnoDB DEFAULT CHARSET=latin11 row in set (0.00 sec)
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---在此表中empno 为主键，在deptno上建有索引
mysql> select * from emp;
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+-------+--------+-----------+------+------------+------+------+--------+| empno | ENAME | JOB | MGR | HIRE | SAL | COMM | deptno |+-------+--------+-----------+------+------------+------+------+--------+| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800 | NULL | 20 |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600 | 300 | 30 |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250 | 500 | 30 |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975 | NULL | 20 |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250 | 1400 | 30 |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850 | NULL | 30 |
| 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450 | NULL | 10 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-07-13 | 3000 | NULL | 10 |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000 | NULL | 10 |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500 | 0 | 30 |
| 7876 | ADAMS | CLERK | 7788 | 1987-06-13 | 1100 | NULL | 20 |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950 | NULL | 30 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000 | NULL | 20 |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300 | NULL | 10 |
+-------+--------+-----------+------+------------+------+------+--------+14 rows in set (0.00 sec)
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1）全表扫描查询
mysql> explain select * from emp \G
*************************** 1. row ***************************
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id: 1
select_type: SIMPLE
table: emp
type: ALL
possible_keys: NULL key: NULL
key_len: NULL
ref: NULL
rows: 14
Extra: NULL
1 row in set (0.00 sec)
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2、通过主键查询
mysql> explain select * from emp where empno=7788\G
*************************** 1. row ***************************
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id: 1
select_type: SIMPLE
table: emp
type: const
possible_keys: PRIMARY key: PRIMARY
key_len: 4
ref: const
rows: 1
Extra: NULL
1 row in set (0.02 sec)
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3）忽略主键索引
mysql> explain select * from emp ignore index(pri) where empno=7788\G
*************************** 1. row ***************************
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id: 1
select_type: SIMPLE
table: emp
type: ALL
possible_keys: NULL key: NULL
key_len: NULL
ref: NULL
rows: 14
Extra: Using where
1 row in set (0.00 sec)
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4）强制使用主键
mysql> explain select * from emp force index(pri) where empno=7788\G
*************************** 1. row ***************************
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id: 1
select_type: SIMPLE
table: emp
type: const
possible_keys: PRIMARY key: PRIMARY
key_len: 4
ref: const
rows: 1
Extra: NULL
1 row in set (0.00 sec)
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5）通过索引deptno查询
mysql> explain select * from emp where deptno=10\G
*************************** 1. row ***************************
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id: 1
select_type: SIMPLE
table: emp
type: ref
possible_keys: deptno key: deptno
key_len: 5
ref: const
rows: 4
Extra: NULL
1 row in set (0.00 sec)
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6）忽略索引的使用
mysql> explain select * from emp ignore index(deptno) where deptno=10\G
*************************** 1. row ***************************
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id: 1
select_type: SIMPLE
table: emp
type: ALL
possible_keys: NULL key: NULL
key_len: NULL
ref: NULL
rows: 14
Extra: Using where
1 row in set (0.00 sec)
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7）强制使用索引
mysql> explain select * from emp force index (deptno) where deptno=10\G
*************************** 1. row ***************************
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id: 1
select_type: SIMPLE
table: emp
type: ref
possible_keys: deptno key: deptno
key_len: 5
ref: const
rows: 4
Extra: NULL
1 row in set (0.00 sec)
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