SQL NULL Values
In SQL, NULL
is a special marker used to indicate that a data value does not exist in the database. A NULL value
is not the same as a zero value or an empty string. Instead, it represents the absence of a value or
the lack of
data.
When a column is
created in
a database table, it can be defined to allow or disallow NULL values. If a column is
defined as
NOT NULL, then it cannot contain NULL values. If a column is
defined as
NULL, then it can contain NULL values.
Some common uses of NULL
values in SQL include:
· To represent missing or unknown data
· To represent optional data that may or may not be present
· To represent the result of an operation that cannot be
determined or
calculated
To check
for NULL values in
SQL, you can use the IS NULL
or IS NOT NULL
operators.
For example, the following SQL statement will retrieve all rows
from the employees table where the salary column contains a NULL value:
SELECT *
FROM employees
WHERE salary IS NULL;
Conversely, the following SQL statement will retrieve all rows
from the employees table where the salary column does not contain a NULL value:
SELECT *
FROM employees
WHERE salary IS NOT
NULL;
When performing calculations or comparisons involving NULL values, the result is always NULL.
For example, the following SQL statement will return NULL
because one of
the operands is
NULL:
SELECT NULL
+ 1;
To handle NULL values in
calculations or
comparisons,
you can use
the COALESCE function, which returns the first non-NULL value in a list of expressions. For example, the following SQL
statement will
return the value of the salary column, or
0 if the column contains a NULL value:
SELECT COALESCE(salary, 0) FROM
employees;
This statement will replace any NULL values in the salary column with 0.
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