MySQL linked table query, common functions, aggregate functions

mysql Joint table query 、 Common functions 、 Aggregate functions

1. Even the table query

matters needing attention ：
1. Join table query is because the data you need to get is in two tables , A table cannot meet our needs

example ： An employee table , There are employee names and employees id Two fields , Another table is the payroll, which contains employees id Now my requirement is to get the name and corresponding salary of each employee in the company , At this time, we need to use the linked table query –>

2. Join table query is to connect two tables through a certain relationship , If there is no valid connection , It will happen –>

Cartesian product phenomenon –>

surface a Yes m Row data , surface b Yes n That's ok –>

select * from a , b where 1=1–>

1=1 It's an ineffective connection because 1 Is equal to 1 All this is invalid connection –>

shortcoming ：（ No advantages ）–>

1. It may cause Caton ,–>

If the table a Yes 10000 Data , surface b Also have 10000 Data, then if we define invalid connection conditions at this time, it will be queried and displayed 1 Ten million pieces of data –>

2. Trouble finding demand data –>

For example, the employee's name and salary I mentioned above , If no valid connection is established, then , One employee corresponds to multiple salaries, so we can't get the data we want –>

Connection classification ：

Internal connection ：
Equivalent connection ：
Non equivalent connection
Self join
External connection ：
The left outer join
Right connection
Full outer join

Cross connect

There are many connections, so I'll pick some commonly used ones

Equivalent connection

① The result of equivalent connection of multiple tables is the intersection part of multiple tables
②n Table joins , Need at least n-1 Connection conditions
③ There is no requirement for the order of multiple tables
④ Generally, you need to alias a table
⑤ It can be used with all the clauses mentioned above , Sorting for example 、 grouping 、 Screening

example ： Query the corresponding salary of each employee

select name,money from yuangong , gomgzi where yuangong.id=gomgzi.id

Here we can use alias to make the code look simpler

select name,money from yuangong y , gomgzi g where y.id=g.id

External connection

1、 The query result of the external connection is all records in the main table. If there are matching records from the table , If there is no matching value from the table , Is displayed null External connection query results = Internal connection result + Records in the master table but not in the slave table
2、 The left outer join ,left join On the left is the main watch Right connection ,right join On the right is the main watch
3、 Left outer and right outer exchange the order of two tables , Can achieve the same effect
4、 Full outer join = The result of internal connection + surface 1 Yes, but the watch 2 There is no the + surface 2 Yes, but the watch 1 There is no the
example

select * from Main table a left outer join From the table b on a.id=b.id where a.id is not null

select a.*,b.* from usera a left outer join grade b on a.id=b.uid where a.id is not null

Common function

Character functions

effect	function	result
Turn lowercase	LOWER(‘SQL Course’)	sql course
Turn capitalization	UPPER(‘SQL Course’)	SQL COURSE
Splicing	CONCAT(‘Hello’, ‘World’)	HelloWorld
Intercept	SUBSTR(‘HelloWorld’,1,5)	Hello
length	LENGTH(‘HelloWorld’)	10
Character occurrence index value	INSTR(‘HelloWorld’, ‘W’)	6
Second half of character interception	TRIM(‘H’ FROM ‘HelloWorld’)	elloWorld
Character substitution	REPLACE(‘abcd’,‘b’,‘m’)	amcd

Number function

effect	function	result
rounding	ROUND(45.926, 2)	45.93
truncation	TRUNC(45.926, 2)	45.92
Seeking remainder	MOD(1600, 300)	100

Date function

effect	function	result
Get current date	now()	Current date time
Convert the character of date format to the date of specified format	STR_TO_DATE(‘9-13-1999’,’%m-%d-%Y’)	1999-09-13
Convert date to character	DATE_FORMAT(‘2018/6/6’,‘%Y year %m month %d Japan ’)	2018 year 06 month 06 Japan

Aggregate functions

1、 SUM()： Sum up .

2、 AVG()： averaging .

3、MAX()： For maximum .

4、 MIN()： For the minimum .（1、2、3、4、 Often with GROUP BY Use it together , It can also be used alone ）

5、 COUNT()： Count the number of records . Often with GROUP BY Use it together , It can also be used alone If there are filter conditions plus keywords having

Merge

1、UNION： Merge all the query results , Then remove the same records

2、UNION ALL： Merge all the query results , The same records will not be removed