When dealing with NULL values in SQL, you need to pay attention to the following methods: 1. Use IS NULL and IS NOT NULL for judgment, and cannot use = NULL or != NULL; 2. Use COALESCE function to replace the NULL value and return the first non-NULL parameter, which is suitable for display and calculation scenarios; 3. Use CASE WHEN to implement more complex logical processing, such as classification according to different situations; 4. Avoid accidental NULL generation during insertion and update. It is recommended to add NOT NULL constraints to the key fields, set default values, and check data integrity before insertion. Correct handling of NULL can reduce query errors and potential bugs.
It is actually quite common to deal with NULL values in SQL, but many people are prone to confusion at the beginning. Simply put, NULL represents a value of "unknown" or "missing", which is not 0, nor an empty string, but a special state. If handled incorrectly, it may cause the query result to be incorrect or incomplete.
The following is a few actual scenarios and talk about common methods and suggestions.
Use IS NULL and IS NOT NULL to judge
In SQL queries, you cannot use = NULL or != NULL to determine whether it is a null value. IS NULL or IS NOT NULL must be used.
For example, if you want to find a record with a field empty:
SELECT * FROM users WHERE email IS NULL;
In turn, look for non-empty ones:
SELECT * FROM users WHERE email IS NOT NULL;
This is a very basic but easy to get stuck. Many novices will mistakenly write WHERE email = NULL , so that no results can be found.
COALESCE function: Replace NULL value
Sometimes we want to replace NULL with a default value when displaying or calculating, and we can use the COALESCE function.
It accepts multiple parameters and returns the first non-NULL value. For example:
SELECT name, COALESCE(phone, 'no contact information') AS contact FROM customers;
In this example, if the phone is NULL, "No contact information" will be displayed.
This function is also often used for numerical operations, such as replacing NULL before finding the average:
SELECT AVG(COALESCE(score, 0)) FROM students;
This way, even if some scores are missing, the entire average value can be avoided from becoming NULL.
CASE WHEN handles more complex logic
If you need to do different processing based on whether it is NULL, you can combine it with CASE WHEN statement.
For example, I want to give user status prompts according to different situations:
SELECT name,
CASE
WHEN last_login IS NULL THEN 'Not logged in'
ELSE 'Login'
END AS login_status
FROM users;This method is more flexible and is suitable for reporting, classified statistics and other scenarios.
Be careful to avoid accidental NULL generation when inserting and updating
When creating a table, it is best to clarify which fields are allowed to be NULL and which are not allowed. If a field is not assigned to the value when inserting data and the default value is not set, NULL may be accidentally inserted.
for example:
INSERT INTO orders (user_id, amount) VALUES (1, NULL);
This will cause the amount field to be NULL, and there may be problems in subsequent statistics.
So suggestion:
- Add
NOT NULLconstraints to key fields - Set reasonable default values (such as
DEFAULT 0) - Check data integrity before insertion
Basically that's it. Although NULL seems simple, it has a great impact in actual development, especially when it comes to joining, grouping, and aggregation calculations. Paying more attention to these details can reduce many bugs.
The above is the detailed content of How to handle NULL values in SQL. For more information, please follow other related articles on the PHP Chinese website!
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