mysql query merge

MySQL query merging refers to merging the results of multiple queries into a result set. This query method is usually used for the purpose of merging data from multiple tables together for comprehensive analysis and reporting.

In MySQL, we can use the UNION operator to achieve query merging. The UNION operator is used to combine the result sets of two or more SELECT statements. It will remove duplicate records and keep only distinct records. The basic syntax of the UNION operator is as follows:

SELECT column1, column2, ... FROM table1 UNION SELECT column1, column2, ... FROM table2;

In the above syntax, we can see that in the two SELECT statements, we need to specify the column to be queried and the table to be queried. Use the UNION operator to combine the two query results and remove duplicate records.

In addition to the basic syntax, we can also use the UNION ALL operator to merge query results and retain duplicate records. The syntax of the UNION ALL operator is similar to the UNION operator, just replace UNION with UNION ALL.

In addition, we can also include an ORDER BY statement in the UNION statement to sort the merged result set. The ORDER BY clause can be placed after the last SELECT statement or at the end of the entire UNION statement.

At the same time, we can also use subquery statements in UNION statements to achieve more flexible queries. For example:

SELECT id, name FROM table1 UNION SELECT id, name FROM ( SELECT id, name FROM table2 WHERE date >= '2020-01-01' ) AS t;

In the above statement, we use the subquery statement to filter out the records with a date greater than or equal to January 1, 2020 in the table2 table, and query and merge them as part of the UNION statement.

In short, MySQL query merging is a very useful query technique that can help us better analyze and report data from multiple tables. By using the UNION operator, we can easily combine multiple query results into a single result set, sort and filter them. At the same time, using subquery statements allows for more flexible query merging, helping us better solve complex data analysis problems.

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