MySQL SQLLesson 13Free preview

MySQL NULL Values

This lesson is part of MySQL · All levels

Summary

The value NULL means data is unknown or unfilled, not equal to zero or empty string. Because of this particular property, comparing NULL with the operator = or <> will always produce UNKNOWN so that the line does not appear. To check NULL, use the special clauses IS NULL and IS NOT NULL.

Key points

NULL is an unknown value marker, different from 0, '', and spaces
Comparisons such as columns = NULL never TRUE; mandatory use of IS NULL
Columns that allow NULL should not be given a PRIMARY KEY
Use DEFAULT or NOT when each row must always have a value

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