What is the difference between union and union all

UNION The operator is used to merge two or more SELECT The result set of the statement .
Please note that ,UNION Inside SELECT Statements must have the same number of columns . Columns must also have similar data types . meanwhile , Every one of them SELECT The order of the columns in the statement must be the same .

Union： Combine two result sets , Do not include repeating lines , At the same time, sort the default rules ;
UNION Duplicate records will be filtered out after table linking , Therefore, after the table is linked, the generated result set will be sorted , Delete duplicate records and return results .
In practice, most applications will not produce duplicate records , The most common are process tables and history tables UNION

Union All： Combine two result sets , Include repeating lines , No sorting ;
If there are duplicate data in two returned result sets , Then the returned result set will contain duplicate data

example

The query results of the two tables are as follows

It's easy to see , Among the test data inserted above , One is repeated

Then we Let's look at the execution first union have a look

Re execution union all have a look