MySQL join skills

The cartesian product ：CROSS JOIN

Understand all kinds of JOIN First of all, we need to understand Cartesian product . Cartesian product is to A Each record in the table is related to B Each record of the table is forcibly put together . therefore , If A Table has n Bar record ,B Table has m Bar record , The result of the Cartesian product will be n*m Bar record . The following example ,t_blog Yes 10 Bar record ,t_type Yes 5 Bar record , All their Cartesian products have 50 Bar record . There are five ways to produce Cartesian product .

 

Internal connection ：INNER JOIN

Internal connection INNER JOIN Is the most commonly used connection operation . From a mathematical point of view, it is to find the intersection of two tables , From the perspective of Cartesian product, it is to pick out ON Record with Clause condition . Yes INNER JOIN,WHERE（ Equivalent connection ）,STRAIGHT_JOIN,JOIN( Omit INNER) Four ways of writing . As for which kind of good I will be MySQL Of JOIN（ Two ）： Optimized narration . Examples are as follows .

 

Left connection ：LEFT JOIN

Left connection LEFT JOIN It means to find the intersection of two tables plus the remaining data in the left table . Still from the Cartesian product point of view , That is to pick out... From Cartesian product first ON Record with Clause condition , Then add the remaining records in the left table （ See the last three ）.

 

The right connection ：RIGHT JOIN

The same goes for right connection RIGHT JOIN Is to find the intersection of two tables plus the remaining data of the right table . Again, from the perspective of Cartesian product , Right join is to pick out... From Cartesian product ON Record with Clause condition , Then add the remaining records in the right table （ See the last one ）.

 

External connection ：OUTER JOIN

Outer connection is to find the union of two sets . From the perspective of Cartesian product, it is to pick out ON Record with Clause condition , Then add the remaining records in the left table , Finally, add the remaining records in the right table . in addition MySQL I won't support it OUTER JOIN, But we can do the result of left connection and right connection UNION Operation to achieve .

 

USING Clause

MySQL Middle link SQL In the sentence ,ON The syntax form of clause is ：table1.column_name = table2.column_name. When the schema design uses the same naming style for the columns of the join table , You can use USING Grammar to simplify ON grammar , The format is ：USING(column_name). 

therefore ,USING The function of is equivalent to ON, The difference lies in USING Specify an attribute name to connect two tables , and ON Specify a condition . in addition ,SELECT * when ,USING Will be removed USING Specified column , and ON Can't . Examples are as follows .

 

Natural join ：NATURE JOIN

Natural connection is USING A simplified version of the clause , It finds the same columns in the two tables as the join condition to join . There is a left natural connection , Right natural connection and ordinary natural connection . stay t_blog and t_type Example , The same columns of the two tables are id, So I'll take it id As a condition of connection . 

In addition, be sure to distinguish between the following three sentences .

Natural join :SELECT * FROM t_blog NATURAL JOIN t_type; 

The cartesian product :SELECT * FROM t_blog NATURA JOIN t_type; 

The cartesian product :SELECT * FROM t_blog NATURE JOIN t_type;