May 2020
Beginner
564 pages
14h 9m
English
Content preview from Learn SQL Database Programming
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Unique constraints
To ensure that all values in a column or columns are different from each other, you use a unique constraint. This type of key can be applied to any data type and is used to avoid duplicate data. You can apply a unique constraint to multiple columns so that it creates a unique value across those multiple columns. It can contain null values.
If you create a unique constraint on one column, it will force the table to have unique values in that specific column. If they are not unique, then the row will not be able to be inserted or updated.
In the following screenshot, the parkkey constraint is unique. All the other fields can have duplicate information as long as the parkkey unique constraint isn't violated:
If you create ...
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