The ALTER command in SQL is used to modify the structure of an existing table. It allows you to add, delete, or modify columns, as well as add or drop constraints.
> 原文:[https://www . geesforgeks . org/SQL-query-add-unique-key-constraints-using-alter-command/](https://www.geeksforgeeks.org/sql-query-to-add-unique-key ...
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