MySQL (11) DQL Joint Query

First, meaning

Union: Merging, combining, merging multiple query results into one result

Second, the grammar

Query Statement 1 Union "all" query Statement 2 Union "all" ...

Third, significance

1. Split a more complex query statement into multiple statements

2, for querying multiple tables, the query column is basically the same

Iv. Features

1. The number of query columns that require multiple query statements must be consistent

2, the query requires multiple query statements of the column type, the order of best consistent

3, Union de-weight, union all contains duplicates

V. Examples

Case 1: Query the department number >90 or the mailbox contains employee information for a

SELECT * FROM Employees WHERE email like '%a% '

UNION

SELECT * FROM Employees WHERE department_id>90;

Case 2: Querying information about men in China and users of middle-aged males in foreign users

SELECT id,cname from T_ca WHERE csex= ' man '

UNION All

SELECT t_id,tname from T_ua WHERE tgender= ' male ';

Vi. Summary of Inquiries

Grammar:

Select query List ⑦

From table 1 alias ①

Connection type join table 2 ②

On connection Condition ③

where filter ④

Group by group List ⑤

Having screening ⑥

Order by sorted list ⑧

Limit Start Entry index, number of entries; ⑨

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MySQL (11) DQL Joint Query