NULL in MySQL represents an unknown value and cannot be judged by the normal comparison symbol. IS NULL or IS NOT NULL must be used. 1. When NULL participates in comparison, it returns UNKNOWN and will not be accepted as TRUE by the WHERE condition; 2. Aggregation functions such as SUM and AVG will ignore NULL, COUNT(*) counts all rows, COUNT(column) does not count NULL; 3. When handling NULL, COALESCE() or IFNULL() can be used to replace the default value; 4. When creating tables, the key fields should be set to NOT NULL; 5. Special attention should be paid to the impact of NULL in JOIN and WHERE conditions.
When processing NULL values ??in MySQL, many people will find that they are different from ordinary logical judgments. This is because MySQL uses three-valued logic, and in addition to TRUE and FALSE, there is also a UNKNOWN state. And NULL is the key factor in this "unknown" state.

What is NULL? Why is it so special?
In the database, NULL represents missing values ??or unknown data , rather than zero, empty strings, or false. This causes you to be unable to judge like normal values ??when performing comparison operations.

For example:
SELECT * FROM users WHERE name = NULL;
This statement will not return any results, because the result of NULL = NULL
is not TRUE, but UNKNOWN. To correctly determine whether it is NULL, IS NULL
or IS NOT NULL
should be used:

SELECT * FROM users WHERE name IS NULL;
How does three-value logic affect query results?
MySQL's boolean expressions can return three results: TRUE, FALSE, and UNKNOWN. This means that some logical rules you may be used to no longer apply.
For example:
SELECT * FROM orders WHERE status != 'completed';
If there is a NULL value in status
column, these rows will not be included in the result , because the result of NULL != 'completed'
is UNKNOWN, not TRUE.
So if you want to include records with unknown status at the same time, you have to add additional judgment:
SELECT * FROM orders WHERE status != 'completed' OR status IS NULL;
How is NULL handled in aggregate functions?
Many aggregate functions such as SUM()
, AVG()
, and COUNT(column)
will automatically ignore the NULL value. But sometimes it is easy to get confused.
for example:
-
COUNT(*)
: counts all rows, including NULL. -
COUNT(name)
: Only count rows whose name is not NULL. -
SUM(score)
: Items with score NULL will be skipped.
So when writing report SQL, pay attention to what you want to count. If you accidentally use the wrong column name, the data may be missed.
How to handle NULL safely?
To avoid the trouble caused by NULL, you can consider the following methods:
Use the COALESCE() function
It can replace NULL with a default value:
SELECT COALESCE(phone, 'no phone') FROM customers;
Use IFNULL() to simplify judgment
Only suitable for two parameters:
SELECT IFNULL(price, 0) FROM products;
-
Try to avoid having the key fields NULL when creating tables
If certain fields must have values, setting to
NOT NULL
can reduce many problems. -
Note the NULL in the JOIN and WHERE conditions
For example, the right table field after LEFT JOIN may be NULL. You should be extra careful when making conditional judgments.
Basically that's it. NULL seems simple, but if you are not careful, you will not be able to find the data, calculate the total number incorrectly, and even write SQL that never meets the conditions. Understanding the relationship between its behavior and three-value logic can help you avoid many pitfalls.
The above is the detailed content of Working with NULL and Three-Valued Logic in MySQL. For more information, please follow other related articles on the PHP Chinese website!

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