Detailed explanation of the implementation method of multi-table non-correlated query in mysql

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When you use

MySQL to query, it is usually a direct query of one table, or it is a related query of multiple tables. Left join (left join), right join (right join), inner join (inner join), and outer join (outer join) are used. This kind of relationship has a certain relationship between the two tables, which is what we often say has a foreign key corresponding relationship, which can be written using the statement a.id = b.aId. This is commonly used by everyone, but sometimes we need to query two or more tables at the same time, and these tables are not related to each other. For example, if we want to query certain data in the user table and user_history table, This time is the so-called non-correlated query.

The

union all statement is used at this time. For example:

(select name,sex,age from user where name like '王%' ) union all (select name,sex,age from user_history where name like '王%' ) ;

This statement is used to query the information of all persons with the surname Wang in the user table and history table. This can also be sorted and intercepted.

(select name,sex,age from user where name like '王%' ) union all (select name,sex,age from user_history where name like '王%' ) order by age desc limit 0,50;

This is to get the top 50 people sorted by age in these two tables.

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