SQL: 2 books in 1: Advanced and Elite Level SQL From The Ground Up
We are aware of how to work with the rows of a table. So far, we have been using queries that work on one row at a time. Now, I will introduce the concept of Set Operations which will allow you to combine the results of multiple queries in one result set. In SQL, there are three set operators:
UNION and UNION ALL
UNION clause is required to combine the results of two or more select statements in a single table. However, it is important that the results of both the queries have same number of columns with compatible data types or else it will not be possible to achieve union results. What you need to know here is that when you use the UNION clause there is no surety about the order in which the rows will appear in the result set. Hence, if you require rows to appear in a specific order then it is important to use ORDER BY clause. For faster results you can use UNION ALL.
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