62. Set operators: UNION and UNION ALL

Set operators have roots in the actual branch of mathematics – set algebra.

In the SQL context a set is a result of a query. Basic set operations are unions and intersections. They are available to us in the form of 4 operators: UNION, UNION ALL, EXCEPT and INTERSECT. Let’s have a closer look at the first two.

UNION

UNION combines the results of 2 SQL queries together. There are a bunch of important features of UNION:

  • result sets must have the same number of columns ⚠
  • column types must match ⚠
  • it filters out duplicated rows. The result of UNION...
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