Column Constraints
When you create a PostgreSQL table, you can define column constraints[15]. A column constraint is a rule that must be satisfied whenever you insert or update a value in that column.
[15] You can also define table constraints. A table constraint applies to the table as a whole, not just a single column. We'll discuss table constraints in Chapter 3, “PostgreSQL SQL Syntax and Use.”
It's very important to understand that when you define a column constraint, PostgreSQL won't ever let your table get into a state in which the constraints are not met. If you try to INSERT a value that violates a constraint, the insertion will fail. If you try to UPDATE a value in such a way that it would violate a constraint, the modification will ...
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