In mysql, union is used to combine the results of multiple select statements into one result set and delete duplicate data in the result set. The syntax is "select column,...from table1 union select column,.. .from table2".
The operating environment of this tutorial: windows10 system, mysql8.0.22 version, Dell G3 computer.
What is the usage of union in mysql
1.mysql union syntax
mysql union is used to combine data from multiple select statements The results are combined into a result set. The syntax is:
select column,......from table1 union [all] select column,...... from table2 ...
In multiple select statements, the corresponding columns should have the same field attributes, and the field name used in the first select statement is also used for the field name of the result.
1.1 The difference between union and union all
When using union, mysql will delete duplicate records in the result set, while using union all, mysql will delete all Records are returned, and the efficiency is higher than union.
2. mysql union usage examples
union is commonly used to query two or more tables similar to data, such as different data classification tables, or data history tables Wait, here are the two tables used for testing;
2.1 Use UNION query
to query the article id numbers in the two tables and titles, and remove duplicate records:
SELECT aid,title FROM article UNION SELECT bid,title FROM blog
The returned query results are as follows:
2.2.UNION query result description
Duplicate records refer to records in which each field in the query is completely repeated. As in the above example, if the title is the same but the id number is different, it is counted as a different record.
The field name used in the first SELECT statement is also used for the field name of the result, such as aid in the above example.
The field names of each SELECT statement can be different, but the field attributes must be consistent.
3. Use UNION ALL query
to query the article ID numbers and titles in the two tables, and return all records:
SELECT aid,title FROM article UNION ALL SELECT bid,title FROM blog
The returned query results are as follows:
Obviously, when using UNION ALL, you simply combine the various queries together without judging whether the data is repeated. Therefore, when it is determined that there will be no duplicate data in the query results or there is no need to remove duplicate data, UNION ALL should be used to improve query efficiency.
Recommended learning: mysql video tutorial
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