What is the usage of union all in mysql

In mysql, the usage of union all is "select statement 1 union all select statement 2...", union all can combine the results from multiple select statements into one result set, and combine all Record return, this efficiency will be higher than union.

The operating environment of this tutorial: windows10 system, mysql8.0.22 version, Dell G3 computer.

What is the usage of union all in mysql

1.mysql union syntax

mysql union is used to combine the results from multiple select statements 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, but using union all, mysql will return all records, and it is efficient 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, etc. The following is used Two tables tested;

2.1 Use UNION query

to query the article ID numbers and titles in the two tables, 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 will be counted as different records.

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, each query is simply combined together without judging whether the data is repeated. Therefore, when you are sure that there will be no duplicate data in the query results or there is no need to remove duplicate data, you should use UNION ALL to improve query efficiency.

Recommended learning: mysql video tutorial

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