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Per the postgresql documentation, Enumerated (enum) types are data types that comprise a static,
ordered set of values. They are equivalent to the enum types supported in a number of programming
languages. An example of an enum type might be the days of the
week, or a set of status values for a piece of data. In this example, we are going to create an enumerated type "rainbow" with specific allowed colors.
Now if we try to insert an invalid type, we will trigger an error:
The sql except operator returns rows that are in the first selection but not in the second selection. Following on with our like example, assume we want all countries with "New" in their name, but not countries with "Zealand" in their names.
Exists is used to run one subquery when a second subquery returns at least one row. For example: