mysql> INSERT INTO tbl_dept(deptName,locAdd) VALUES('技术部',11);Query OK, 1 row affected (0.07 sec)
mysql> INSERT INTO tbl_dept(deptName,locAdd) VALUES('美工部',12);Query OK, 1 row affected (0.08 sec)
mysql> INSERT INTO tbl_dept(deptName,locAdd) VALUES('总裁办',13);Query OK, 1 row affected (0.06 sec)
mysql> INSERT INTO tbl_dept(deptName,locAdd) VALUES('人力资源',14);Query OK, 1 row affected (0.11 sec)
mysql> INSERT INTO tbl_dept(deptName,locAdd) VALUES('后勤组',15);Query OK, 1 row affected (0.10 sec)
mysql> insert into tbl_emp(name,deptId) values('jack',1);Query OK, 1 row affected (0.11 sec)
mysql> insert into tbl_emp(name,deptId) values('tom',1);Query OK, 1 row affected (0.08 sec)
mysql> insert into tbl_emp(name,deptId) values('alice',2);Query OK, 1 row affected (0.08 sec)
mysql> insert into tbl_emp(name,deptId) values('john',3);Query OK, 1 row affected (0.13 sec)
mysql> insert into tbl_emp(name,deptId) values('faker',4);Query OK, 1 row affected (0.10 sec)
mysql> insert into tbl_emp(name) values('mlxg');Query OK, 1 row affected (0.13 sec)
mysql> select * from tbl_dept;+----+----------+--------+| id | deptName | locAdd |+----+----------+--------+| 1 | 技术部 | 11 || 2 | 美工部 | 12 || 3 | 总裁办 | 13 || 4 | 人力资源 | 14 || 5 | 后勤组 | 15 |+----+----------+--------+5 rows in set (0.00 sec)
mysql> select * from tbl_emp;+----+-------+--------+| id | name | deptId |+----+-------+--------+| 1 | jack | 1 || 2 | tom | 1 || 3 | alice | 2 || 4 | john | 3 || 5 | faker | 4 || 7 | ning | NULL || 8 | mlxg | NULL |+----+-------+--------+7 rows in set (0.00 sec)
两表的关联关系如图所示：
1、左连接(A独有+AB共有)
查询所有部门以及各部门的员工数：
mysql> select t1.id,t1.deptName,count(t2.name) as emps from tbl_dept t1 left join tbl_emp t2 on t2.deptId=t1.id group by deptName order by id;+----+----------+------+| id | deptName | emps |+----+----------+------+| 1 | 技术部 | 2 || 2 | 美工部 | 1 || 3 | 总裁办 | 1 || 4 | 人力资源 | 1 || 5 | 后勤组 | 0 |+----+----------+------+5 rows in set (0.00 sec)
2、右连接(B独有+AB共有)
查询所有员工及其所属部门：
mysql> select t2.id,t2.name,t1.deptName from tbl_dept t1 right join tbl_emp t2 on t2.deptId=t1.id;+----+-------+----------+| id | name | deptName |+----+-------+----------+| 1 | jack | 技术部 || 2 | tom | 技术部 || 3 | alice | 美工部 || 4 | john | 总裁办 || 5 | faker | 人力资源 || 7 | ning | NULL || 8 | mlxg | NULL |+----+-------+----------+7 rows in set (0.04 sec)
3、内连接(AB共有)
查询两表共有的数据：
mysql> select deptName,t2.name empName from tbl_dept t1 inner join tbl_emp t2 on t1.id=t2.deptId;+----------+---------+| deptName | empName |+----------+---------+| 技术部 | jack || 技术部 | tom || 美工部 | alice || 总裁办 | john || 人力资源 | faker |+----------+---------+
4、A独有
即在(A独有+AB共有)的基础之上排除B即可(通过b.id is null即可实现)：
mysql> select a.deptName,b.name empName from tbl_dept a left join tbl_emp b on a.id=b.deptId where b.id is null;+----------+---------+| deptName | empName |+----------+---------+| 后勤组 | NULL |+----------+---------+
5、B独有
与(A独有)同理：
mysql> select a.name empName,b.deptName from tbl_emp a left join tbl_dept b on a.deptId=b.id where b.id is null;+---------+----------+| empName | deptName |+---------+----------+| ning | NULL || mlxg | NULL |+---------+----------+
6、A独有+B独有
使用union将(A独有)和(B独有)联合在一起：
mysql> select a.deptName,b.name empName from tbl_dept a left join tbl_emp b on a.id=b.deptId where b.id is null union select b.deptName,a.name emptName from tbl_emp a left join tbl_dept b on a.deptId=b.id where b.id is null;+----------+---------+| deptName | empName |+----------+---------+| 后勤组 | NULL || NULL | ning || NULL | mlxg |+----------+---------+
7、A独有+AB公共+B独有
使用union(可去重)联合(A独有+AB公共)和(B独有+AB公共)
mysql> select a.deptName,b.name empName from tbl_dept a left join tbl_emp b on a.id=b.deptId union select a.deptName,b.name empName from tbl_dept a right join tbl_emp b on a.id=b.deptId;+----------+---------+| deptName | empName |+----------+---------+| 技术部 | jack || 技术部 | tom || 美工部 | alice || 总裁办 | john || 人力资源 | faker || 后勤组 | NULL || NULL | ning || NULL | mlxg |+----------+---------+
mysql> select * from student;+----+-----------+------+| id | stuId | name |+----+-----------+------+| 1 | 123456789 | jack || 3 | NULL | tom |+----+-----------+------+
索引字段为char类型 + not null：key_len = 字段申明字符个数 * 3(utf8编码的每个字符占3个字节)
mysql> create table test( -> id int(10) not null auto_increment, -> primary key(id) -> ) engine=innodb auto_increment=1 default charset=utf8;
mysql> alter table test add c1 char(10) not null;
mysql> create index idx_c1 on test(c1);
Using filesort：说明mysql会对数据使用一个外部的索引排序，而不是按照表内的索引顺序进行读取。MySQL中无法利用索引完成的排序操作称为“文件排序”
mysql> explain select * from person order by lastName\G*************************** 1. row *************************** id: 1 select_type: SIMPLE table: person partitions: NULL type: indexpossible_keys: NULL key: idx_name key_len: 186 ref: NULL rows: 1 filtered: 100.00 Extra: Using index; Using filesort
使用\G代替;结尾可以使执行计划垂直显示。
mysql> explain select * from person order by firstName,lastName\G*************************** 1. row *************************** id: 1 select_type: SIMPLE table: person partitions: NULL type: indexpossible_keys: NULL key: idx_name key_len: 186 ref: NULL rows: 1 filtered: 100.00 Extra: Using index
Using temporary：使用了临时表保存中间结果。MySQL在对查询结果聚合时使用临时表。常见于排序 order by 和分组查询 group by。
mysql> insert into person(firstName,lastName) values('张','三');
mysql> insert into person(firstName,lastName) values('李','三');
mysql> insert into person(firstName,lastName) values('王','三');
mysql> insert into person(firstName,lastName) values('李','明');
mysql> select lastName,count(lastName) from person group by lastName;+----------+-----------------+| lastName | count(lastName) |+----------+-----------------+| 三 | 3 || 明 | 1 |+----------+-----------------+
mysql> explain select lastName,count(lastName) from person group by lastName\G*************************** 1. row *************************** id: 1 select_type: SIMPLE table: person partitions: NULL type: indexpossible_keys: idx_name key: idx_name key_len: 186 ref: NULL rows: 4 filtered: 100.00 Extra: Using index; Using temporary; Using filesort
Using index：表示相应的select操作中使用了覆盖索引(Covering Index)，避免访问了表的数据行(需要读磁盘)，效率不错！如果同时出现Using where，表明索引被用来执行索引键值的查找；如果没有同时出现Using where，表明索引用来读取数据而非执行查找动作。
mysql> explain select * from test where c5='' and c7=''\G*************************** 1. row *************************** id: 1 select_type: SIMPLE table: test partitions: NULL type: refpossible_keys: idx_c5_c6_c7 key: idx_c5_c6_c7 key_len: 33 ref: const rows: 1 filtered: 100.00 Extra: Using index condition1 row in set, 1 warning (0.00 sec)
mysql> explain select * from test where c5='' and c7='' and c6=''\G*************************** 1. row *************************** id: 1 select_type: SIMPLE table: test partitions: NULL type: refpossible_keys: idx_c5_c6_c7 key: idx_c5_c6_c7 key_len: 99 ref: const,const,const rows: 1 filtered: 100.00 Extra: NULL1 row in set, 1 warning (0.00 sec)
最优的做法是：
mysql> explain select * from test where c5=''\G
mysql> explain select * from test where c5='' and c6=''\G
mysql> explain select * from test where c5='' and c6='' and c7=''\G
3、不在列名上添加任何操作
有时我们会在列名上进行计算、函数运算、自动/手动类型转换，这会直接导致索引失效。
mysql> explain select * from person where left(firstName,1)='张'\G*************************** 1. row *************************** id: 1 select_type: SIMPLE table: person partitions: NULL type: indexpossible_keys: NULL key: idx_name key_len: 186 ref: NULL rows: 4 filtered: 100.00 Extra: Using where; Using index
mysql> explain select * from person where firstName='张'\G*************************** 1. row *************************** id: 1 select_type: SIMPLE table: person partitions: NULL type: refpossible_keys: idx_name key: idx_name key_len: 93 ref: const rows: 1 filtered: 100.00 Extra: Using index
