Review of MySQL (VI): usage of Union and limit

MySql Review ( 6、 ... and )：Union and Limit Usage of

union

union You can merge collections （ Add up ）

1. use or

select ename,job from emp where job = 'MANAGER' or job = 'SALESMAN';

1. Inquire about job contain MANAGER And contains SALESMAN The employees'

select * from emp where job in('MANAGER', 'SALESMAN');

3. use union To merge

select * from emp where job='MANAGER'
union
select * from emp where job='SALESMAN';

The spliced data must correspond to each other . A table cannot be a data , The other table is two data , This cannot be spliced ！

limit( a key )

It is mainly used to extract the first or middle rows of data ,limit yes mysql Peculiar , There are no other databases , Not universal .（Oracle There's the same mechanism in , be called rownum）

Grammatical mechanism ：

limit startIndex, length
	startIndex Indicates the starting position , from 0 Start ,0 Represents the first data .
	length Take a few 

 Case study ： Before taking out the salary 5 A great employee （ Ideas ： Before descending 5 individual ）
	select ename,sal from emp order by sal desc;
	 Take before 5 individual ：
	select ename,sal from emp order by sal desc limit 0, 5;
	select ename,sal from emp order by sal desc limit 5;

Get the first five data

select * from emp limit 5;

Take two pieces of data from the second one

select * from emp limit 1,2;

Before getting the highest salary 5 name

select * from emp e order by e.sal desc limit 5;

Find the fourth to ninth highest paid employees

select ename,sal from emp order by sal desc limit 3,6;

On the page

1. limit yes sql The last step in the execution of a statement ：

	select		5
		...
	from			1
		...		
	where			2
		...	
	group by		3
		...
	having		4
		...
	order by		6
		...
	limit			7
		...;

2. Common standard paging

 Each page shows 3 Bar record ：
 The first 1 page ：0, 3
 The first 2 page ：3, 3
 The first 3 page ：6, 3
 The first 4 page ：9, 3
 The first 5 page ：12, 3

 Each page shows pageSize Bar record ：
 The first pageNo page ：(pageNo - 1) * pageSize, pageSize

pageSize What is it? ？ It's how many records are displayed per page 
pageNo What is it? ？ Show the page 

java Code {
	int pageNo = 2; //  The page number is 2
	int pageSize = 10; //  Each page shows 10 strip 
	
	limit (pageNo - 1) * pageSize, pageSize
}