How to solve the connection query problem in MySql

Connection query

When performing a multi-table connection query, you need to specify the table to which the field belongs, which can improve query efficiency. If you do not specify the table to which the field belongs, The database will look for this field from each table.

• e. Field name: means taking a field from the emp table

• emp as e: The alias mechanism of the table, you can omit as and write it directly as emp e

• Connection query: It can also be called cross-table query, which requires multiple tables to be associated to query data

• The emp table and the dept table are combined to query the data. Get the employee name from the emp table and the department name from the dept table.

Note: Do When connecting the query, be sure to write the associated conditions to avoid the Cartesian product phenomenon

Classification of the connection query

Classification based on the age of the grammar

SQL92: The syntax that appeared in 1992

• The shortcomings of sql92: the structure is not clear, the connection conditions of the table, and the conditions for further filtering in the later stage are all placed After where

SQL99:The syntax that appeared in 1999 (focus on learning)

• sql99 advantages: table The connection conditions and query conditions are separated. If you need to further filter after the connection, continue to add where later. It will be clearer when connecting multiple tables.

Connect according to the table Classification of methods

Inner join: equijoin, non-equijoin, self-join, inner can be omitted and generally not added

• Table A inner join table B on association conditions

Outer join: left outer join (left join), right outer join (right join), outer can be omitted and generally not added

• Table A left outer join Table B on association condition

• Table B right outer join Table A on association condition

• Left join with The table on the left shall prevail and compared with the table on the right, any equality or inequalities with the left table will be displayed, and the right table will display the conditions that meet the conditions, and the conditions that do not meet the conditions will not be displayed (right join is just the opposite)

• The functions that can be completed by left join can be completed by right join

Full join, the two tables connected are both main tables and can be queried

Cartesian product phenomenon

When two tables are connected for query and no connection conditions are specified, the number of final query results is the product of the number of entries in the two tables. This phenomenon is called: Cartesian product phenomenon (Discovered by Descartes, this is a mathematical phenomenon)

Avoid the Cartesian product phenomenon: add connection conditions when connecting multiple tables, and filter out records that meet this condition

• The number of matches has not decreased during the matching process, but the number of final query results has become less because it is filtered according to the connection conditions

• Through the Cartesian product phenomenon It can be concluded that the more the number of table connections, the lower the efficiency. Try to avoid the number of table connections

inner joins and equivalent joins

because the conditions are equal. So it is called an equijoin (data with equal connection conditions)

Query the department name of each employee and display the employee name and department

emp e and dept d tables To connect, the connection condition is: e.deptno = d.deptno

	--sql92的缺点：结构不清晰，表的连接条件，和后期进一步筛选的条件，都放到了where后面

	select 
		e.ename,d.dname
	from
		emp e, dept d
	where
		e.deptno = d.deptno;
		
	--sql99优点：表连接的条件是独立的，连接之后，如果还需要进一步筛选，再往后继续添加where		
	select 
		e.ename,d.dname
	from
		emp e
	--inner可以省略（带着inner可读性更好）
	(inner) join
		dept d
	on
		e.deptno = d.deptno;

Inner join self-join

Because there is only one table connection, the specific query method is to treat one table as The two tables connect themselves, so it becomes a self-join

Query the employee’s superior leader and require the employee name and the corresponding leader name to be displayed

One table is viewed as two Table, emp e represents the employee table, and emp m also represents the leader table

Connection conditions: e.mgr = m.empno employee’s leader number = leader’s employee number

--SQL92
select e.ename, m.ename from emp e, emp m where e.mgr=m.empno;

--SQL99
select 
	a.ename as '员工名', b.ename as '领导名'
from
	emp e
join
	emp m
on
	e.mgr = m.empno;

Inner join Non-equivalent connection

Because the connection condition is not an equivalence relationship, it is called non-equivalent connection

Displays employee information with a salary greater than 2000, and displays the name of the department to which they belong

--采用 SQL92 语法
select 
	e.ename, e.sal, d.dname 
from 
	emp e, dept d 
where 
	e.deptno=d.deptno and e.sal > 2000;
	
--采用 SQL99 语法
select 
	e.ename, e.sal, d.dname 
from 
	emp e 
(inner) join 
	dept d 
on 
	e.deptno=d.deptno 
where 
	e.sal>2000;

Find out the salary grade of each employee and require the employee name, salary, and salary grade to be displayed

select 
	e.ename, e.sal, s.grade
from
	emp e
(inner) join
	salgrade s
on
	e.sal between s.losal and s.hisal;

Outer connection

Outer connection Connection: In the outer connection, the connection between the two tables creates a primary and secondary relationship. The main query is the data of the primary table, and the secondary table is queried with the association. That is, if the other party has no records matching my primary table, then the default other party is null, outer Keywords can be omitted

• Right outer join: The one with right is a right outer join, which means that the table to the right of the join keyword is regarded as the main table, mainly to combine this All the data in the table are queried, and the table on the left is queried with the association

• Left outer join: The one with left t is a left outer join, which means that the table to the left of the join keyword will be Treat it as the main table, mainly to query all the data in this table, and query the table on the right with the association

**Note: Any right join has a left join Writing method, any left connection has a right connection writing method**

查询每个员工的上级领导，要求显示所有员工的名字和领导名 , 如果某个员工没有领导 , 那么该员工也必须显示出来

--左连接
select 
	a.ename as '员工名', b.ename as '领导名'
from
	emp a
left (outer) join
	emp b
on
	a.mgr = b.empno; 

--右连接
select 
	a.ename as '员工名', b.ename as '领导名'
from
	emp b
right (outer) join
	emp a
on
	a.mgr = b.empno;

内连接和外连接的特点

内连接：A表和B表连接，A和B两张表没有主次关系是平等的 , 查询时只是将能够匹配上连接条件的数据查询出来 , 即如果没有匹配的就查询不出来 , inner关键字可以省略

外连接: 在外连接当中，两张表连接产生了主次关系 , 主要查询的是主表的数据 , 捎带着关联查询次表 , 即如果对方没有记录和我主表匹配 , 那么默认对方是null , outer关键字可以省略

注意: 区分内外连接的办法是通过 right 和 left 关键字 , 不是通过 inner 和 outer 因为它们都可以省略 , 并且外连接的查询结果条数一定是大于等于内连接的查询结果条数

显示员工信息，并显示所属的部门名称

select 
	e.ename,d.dname
from
	emp e
join
	dept d
on
	e.deptno = d.deptno;

显示员工信息，并显示所属的部门名称，如果某一个部门没有员工，那么该部门也必须显示出来

--外连接（右外连接）
select 
	e.ename,d.dname
from
	emp e 
--outer是可以省略的，带着可读性强
--right代表什么：表示将join关键字右边的这张表看成主表，主要是为了将这张表的数据全部查询出来，捎带着关联查询左边的表
right (outer) join 
	dept d
on
	e.deptno = d.deptno;


--外连接（左外连接）
select 
	e.ename,d.dname
from
	dept d 
--outer是可以省略的，带着可读性强
left (outer) join 
	emp e
on
	e.deptno = d.deptno;

多表连接

一条SQL中内连接和外连接可以混合 , 都可以出现

--表示一: a 和 b 进行内连接 , a 和 c 进行内连接 , a 和 d 进行右外连接(推荐)

--表示二: a 和 b 的内连接结果去内连接 c 的结果再去右外连接 d
select 
	...
from
	a
join
	b
on
	a和b的连接条件
join
	c
on
	a和c的连接条件
right join
	d
on
	a和d的连接条件

找出每个员工的部门名称以及工资等级，要求显示员工名、部门名、薪资、薪资等级

	select 
		e.ename,e.sal,d.dname,s.grade
	from
		emp e
	join
		dept d
	on 
		e.deptno = d.deptno
	join
		salgrade s
	on
		e.sal between s.losal and s.hisal;

查询结果

	+--------+---------+------------+-------+
	| ename  | sal     | dname      | grade |
	+--------+---------+------------+-------+
	| SMITH  |  800.00 | RESEARCH   |     1 |
	| ALLEN  | 1600.00 | SALES      |     3 |
	| WARD   | 1250.00 | SALES      |     2 |
	| JONES  | 2975.00 | RESEARCH   |     4 |
	| MARTIN | 1250.00 | SALES      |     2 |
	| BLAKE  | 2850.00 | SALES      |     4 |
	| CLARK  | 2450.00 | ACCOUNTING |     4 |
	| SCOTT  | 3000.00 | RESEARCH   |     4 |
	| KING   | 5000.00 | ACCOUNTING |     5 |
	| TURNER | 1500.00 | SALES      |     3 |
	| ADAMS  | 1100.00 | RESEARCH   |     1 |
	| JAMES  |  950.00 | SALES      |     1 |
	| FORD   | 3000.00 | RESEARCH   |     4 |
	| MILLER | 1300.00 | ACCOUNTING |     2 |
	+--------+---------+------------+-------+

找出每个员工的部门名称以及工资等级，还有上级领导，要求显示员工名、领导名、部门名、薪资、薪资等级

	select 
		e.ename,e.sal,d.dname,s.grade,l.ename
	from
		emp e
	join
		dept d
	on 
		e.deptno = d.deptno
	join
		salgrade s
	on
		e.sal between s.losal and s.hisal
	left join
		emp l
	on
		e.mgr = l.empno;

查询结果

	+--------+---------+------------+-------+-------+
	| ename  | sal     | dname      | grade | ename |
	+--------+---------+------------+-------+-------+
	| SMITH  |  800.00 | RESEARCH   |     1 | FORD  |
	| ALLEN  | 1600.00 | SALES      |     3 | BLAKE |
	| WARD   | 1250.00 | SALES      |     2 | BLAKE |
	| JONES  | 2975.00 | RESEARCH   |     4 | KING  |
	| MARTIN | 1250.00 | SALES      |     2 | BLAKE |
	| BLAKE  | 2850.00 | SALES      |     4 | KING  |
	| CLARK  | 2450.00 | ACCOUNTING |     4 | KING  |
	| SCOTT  | 3000.00 | RESEARCH   |     4 | JONES |
	| KING   | 5000.00 | ACCOUNTING |     5 | NULL  |
	| TURNER | 1500.00 | SALES      |     3 | BLAKE |
	| ADAMS  | 1100.00 | RESEARCH   |     1 | SCOTT |
	| JAMES  |  950.00 | SALES      |     1 | BLAKE |
	| FORD   | 3000.00 | RESEARCH   |     4 | JONES |
	| MILLER | 1300.00 | ACCOUNTING |     2 | CLARK |
	+--------+---------+------------+-------+-------+

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