The mysql union query statement is [query statement 1 union query statement 2]. It is required that the type of each column in the query of multiple query statements should be consistent. The union keyword defaults to deduplication. If you use union all, you can Contains duplicates.
The operating environment of this tutorial: Windows 7 system, mysql version 8.0.22, DELL G3 computer. This method is suitable for all brands of computers.
Related free learning recommendations: mysql video tutorial
Union Query
UNION Union Merge: Combine the results of multiple query statements into one result
Syntax:
查询语句1 union 查询语句2
Application scenarios:
The results to be queried come from multiple tables, and the multiple tables have no direct connection relationship, but the queried information is consistent
Features:
1. The number of query columns in multiple query statements is required to be consistent
2. Each query in multiple query statements is required to be consistent. The type of a column should be consistent with the best
3. The union keyword is deduplicated by default. If union all is used, duplicate items can be included
Introduced cases: Query department number > 90 or the email address contains a Employee information
SELECT * FROM employees WHERE email like '%a%' or department_id >90 SELECT * FROM employees WHERE email like '%a%' UNION SELECT * FROM employees where department_id >90;
Case: Query the information of males among Chinese users and the information of males among foreign users
SELECT id,cname,csex FROM t_ca where csex='男' UNION all SELECT t_id,tName FROM t_ua where tGender='male';
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