SQL Learning Combination query (UNION)

1. Most SQL queries contain only a single SELECT statement that returns data from one or more tables, but SQL also allows multiple queries (multiple SELECT statements) to be executed and the result returned as a query result set. These combined queries are commonly referred to as and or compound queries.

There are two main scenarios in which you need to use a combination query:

(1) Return structure data from a different table in a query

(2) Execute multiple queries against one, return data by one query

2. Use Union

Using Union is simple, all you have to do is give each SELECT statement, and then add the Union keyword between each SELECT statement so that the selected result set can be combined into a single result set and returned. The code is as follows:

SQL Learning Combination query (UNION)