Null in sql | NULL command in SQL - sql - sql tutorial - learn sql




  • The SQL NULL is the term used to represent a missing value.
  • A NULL value in a table is a value in a field that appears to be blank.
  • A field with a NULL value is a field with no value.
  • It is very important to understand that a NULL value is different than a zero value or a field that contains spaces.
  • NULL means that data does not exist.
  • NULL is not equal to 0 or an empty string.
  • Both zero and empty string represent a value, while NULL has no value.
  • Any mathematical operations performed on NULL will result in NULL.
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IS NULL Syntax

SELECT column_names
FROM table_name
WHERE column_name IS NULL;

IS NOT NULL Syntax

SELECT column_names
FROM table_name
WHERE column_name IS NOT NULL;
  • In SQL, NULL means that data does not exist. NULL does not equal to 0 or an empty string.
  • Both 0 and empty string represent a value, while NULL has no value.
  • Any mathematical operations performed on NULL will result in NULL. For example,10 + NULL = NULL
  • Aggregate functions such as SUM, COUNT, AVG, MAX, and MIN exclude NULL values.
  • This is not likely to cause any issues for SUM, MAX, and MIN. However, this can lead to confusion with AVG and COUNT.
Let's take a look at the following example:

Table cost_Data

Store _Name Sales
Store A 300
Store B 200
Store C 100
Store D NULL
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Below are the results for each aggregate function:

SELECT SUM (cost), AVG (cost), MAX (cost), MIN (cost), COUNT (cost)
FROM cost _Date;

Result:

SUM (cost) AVG (cost) MAX (cost) MIN (cost) COUNT (cost)
600 200 300 100 3
  • Note that the AVG function counts only 3 rows (the NULL row is excluded), so the average is 600 / 3 = 200, not 600 / 4 = 150.
  • The COUNT function also ignores the NULL row, which is why COUNT (Sales) = 3.

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