The key methods for dealing with NULL values ??in SQL include: 1. Use IS NULL or IS NOT NULL for correct judgments, and avoid using = NULL or != NULL; 2. Use COALESCE, IFNULL and other functions to avoid result interference when calculating fields to avoid result interference; 3. Pay attention to the difference between COUNT(*) and COUNT(column). Other aggregate functions such as SUM and AVG will automatically ignore NULL; 4. Use COALESCE to deal with display and sorting issues to improve the friendliness and consistency of query results. Mastering these key points can effectively improve the accuracy and reliability of SQL queries.

When writing SQL queries, NULL values ??are an inescapable problem. It is not "0" or "empty string", but a special "unknown value" state. Poor processing can easily lead to inaccurate query results or logical errors. If you want to truly write reliable SQL, you have to understand how to deal with NULL.

Common pitfalls for identifying NULL values

Many people make a mistake at the beginning: directly use = NULL or != NULL to determine whether it is NULL. But in fact, in SQL, NULL cannot be compared with any value (including itself), so writing will always return false.

The correct way to do this is to use IS NULL and IS NOT NULL :

 SELECT * FROM users WHERE email IS NULL;

This is the only standard way to identify NULLs. If you see someone writing something like WHERE column = NULL , you can basically conclude that this query will not return any valid data.

Avoid NULL interference in the calculated field

When you do some operations or splicing fields in the SELECT clause, if one of the fields is NULL, the result of the entire expression will also become NULL. for example:

 SELECT first_name ' ' middle_name ' last_name AS full_name FROM users;

As long as middle_name is NULL, the entire full_name will become NULL (the specific behavior depends on the database system). To avoid this, you can replace NULL with functions like COALESCE() or IFNULL() :

 SELECT first_name COALESCE(' ' middle_name, '') ' ' last_name AS full_name FROM users;

What this means here is that if middle_name is NULL, it will be replaced with an empty string, which will not affect the overall splicing.

Functions vary slightly from database to database:

• MySQL uses IFNULL()
• PostgreSQL and SQL Server use COALESCE()
• Oracle uses NVL()

Therefore, you should pay attention to compatibility when writing such statements.

How an aggregation function handles NULL values

Many people think COUNT(*) and COUNT(column) are the same, but in fact the difference is quite big.

• COUNT(*) counts all rows, regardless of whether there is NULL or not.
• COUNT(column) only counts the number of rows that are not NULL in this column.

For example:

 SELECT COUNT(email), COUNT(*) FROM users;

If some users do not fill in the email, the two values ??will be different.

Other aggregate functions such as SUM() , AVG() , etc. will ignore NULL values ??by default. Therefore, when doing numerical analysis, you should pay special attention to not being "automatically ignored" and confuse logic.

Use COALESCE to handle display or sorting issues

Sometimes you want to make the query results appear more friendly, such as displaying NULL as "N/A" or default value, you can use COALESCE() :

 SELECT name, COALESCE(phone_number, 'N/A') AS phone FROM contacts;

This is very practical when presenting reports or front-end calls. In addition, if you want to sort by a field that may be NULL, you can also use COALESCE to fill the default value to ensure the sorting logic is consistent.

Basically that's it. NULL looks simple, but the impact is very wide in actual query, and if you are not careful, you will lay a pit. Mastering these key points will allow you to write more robust SQL.

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