What is Mysql union query

MySQL union query, "UNION", is a SQL operation used to combine the results of multiple SELECT statements into one result set. It can combine data from multiple tables or views into one result set. And to remove duplicate records, the UNION operation will combine the result sets of multiple SELECT statements in the order of columns. The result set of each SELECT statement must have the same number of columns, and the columns in the same position must have similar data types.

MySQL union query (UNION) is a SQL operation used to combine the results of multiple SELECT statements into a single result set. It can combine data from multiple tables or views into a single result set and remove duplicate records.

The UNION operation will combine the result sets of multiple SELECT statements in the order of columns. The result set of each SELECT statement must have the same number of columns, and the columns in the same position must have similar data types. Through the UNION operation, data in multiple tables can be merged together in the same format to facilitate comparative analysis and processing.

The syntax of the UNION operation is as follows:

SELECT column1, column2, ... FROM table1
UNION [ALL]
SELECT column1, column2, ... FROM table2
[UNION [ALL] SELECT column1, column2, ... FROM table3...];

Among them, the column name in the first SELECT statement will be used as the column name of the result set, and the subsequent SELECT The statement must use the same column names as the first SELECT statement. If the ALL parameter is used, it means that duplicate records are retained; otherwise, duplicate records will be removed.

It should be noted that when performing a UNION operation, the number of columns and data type of each SELECT statement must be the same, otherwise an error will occur. In addition, the UNION operation may affect query performance because it requires sorting and deduplication of multiple result sets. Therefore, when performing UNION operations, you should try to avoid using tables or views with large amounts of data.

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