Related knowledge points about SQL NULL values

NULL values ​​are missing unknown data. By default, table columns can store NULL values. This chapter explains IS NULL and IS NOT NULL operators.

SQL NULL value

If a column in the table is optional, then we can insert new records or update existing records without adding a value to the column. This means that the field will be saved with a NULL value.

NULL values ​​are handled differently than other values.

NULL is used as a placeholder for unknown or inapplicable values.

Comments: NULL and 0 cannot be compared; they are not equivalent.

SQL NULL value processing

Please see the following table:

If the "Address" column in the "Persons" table is available Chosen. This means that if you insert a record without a value in the "Address" column, the "Address" column will be saved with a NULL value.

So how do we test for NULL values?

You cannot use comparison operators to test for NULL values, such as =, .

We must use the IS NULL and IS NOT NULL operators.

SQL IS NULL

How do we select only records with NULL values ​​in the "Address" column?

We must use the IS NULL operator:

SELECT LastName,FirstName,Address FROM Persons
WHERE Address IS NULL

:

This article explains SQL NULL. For more learning materials, please pay attention to the php Chinese website.

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