When handling NULL values ??in MySQL, please note: 1. When designing the table, the key fields are set to NOT NULL, and optional fields are allowed NULL; 2. For query judgment, IS NULL or IS NOT NULL must be used, and = or != cannot be used; 3. You can use IFNULL or COALESCE functions to replace the display default values; 4. Be cautious when using NULL values ??directly when inserting or updating, and pay attention to the data source and ORM framework processing methods. NULL represents an unknown value, which is not equal to any value, including itself. Therefore, be careful when querying, counting, and connecting tables to avoid missing data or logical errors. Rational use of functions and constraints can effectively reduce interference caused by NULL.
It is actually quite common to deal with NULL values ??in MySQL, but many people are prone to confusion at the beginning. Simply put, NULL means "no value" or "unknown value". It is not 0, nor is it an empty string, but a special state.
This article will talk about how to deal with NULL values ??when encountering field design and query so as not to error or miss data.
Should NULL be allowed when designing table structure?
This is a common question: Should the field be set to NULL or NOT NULL .
Generally speaking, if you are sure that this field must have a value every time it inserts data, use NOT NULL . For example, key information such as user mobile phone number and order number.
But if there is indeed no data in some cases, such as the user's middle name, optional notes, etc., then you can consider allowing NULL .
suggestion:
- Try to set the key field to
NOT NULLas much as possible to avoid complicated subsequent queries and judgments. - Optional fields allow
NULL, but be clear about which scenarios will be NULL. - If you want "null value" to take up a clear state, such as distinguishing between "unfilled" and "empty string", then NULL is appropriate.
How to correctly judge NULL value in a query?
Many people make a mistake when they first come into contact with MySQL, which is to use = NULL to judge:
SELECT * FROM users WHERE middle_name = NULL;
This actually shows no results. Because NULL cannot be judged by ordinary comparison operators.
The correct way to write it is to use IS NULL or IS NOT NULL :
SELECT * FROM users WHERE middle_name IS NULL;
Common misoperations:
- In the
WHEREcondition,column != 'value'is used, but the column may contain NULL, resulting in incomplete results. - Use a function to process the fields before making judgments, such as
COALESCE(column, '') = '', although it can solve the problem, it affects the index efficiency.
How to handle NULL display in a query?
Sometimes we don't want the query result to be displayed as NULL, but instead want to replace it with the default value, such as 'N/A' or 0.
MySQL provides several functions that can do this:
-
IFNULL(column, default_value): If the field is NULL, the specified default value is returned. -
COALESCE(value1, value2, ...): Find the first non-NULL value from left to right and return.
For example:
SELECT name, IFNULL(email, 'No email') AS email FROM users;
This will turn NULL into a more friendly prompt.
Tips:
- Before presenting a report or front-end, it is best to use these functions to make a layer of conversion to avoid the word "null" on the page.
- Pay attention to type matching. For example, when using
IFNULL(salary, 0), the salary should be a numeric type, otherwise it may cause type conversion problems.
How to control NULL values ??when inserting and updating?
If a field is not assigned a value when inserting data, its value will become NULL (provided that NULL is allowed).
If you want to force NULL, you can write it directly:
INSERT INTO users (name, middle_name) VALUES ('Alice', NULL);
The same goes for updates:
UPDATE users SET middle_name = NULL WHERE id = 123;
Note:
- When importing data in batches, such as syncing from CSV or other systems, be careful whether the null value in the source data will become NULL.
- When some fields in the ORM framework are empty objects, they may be converted to NULL by default, so you need to pay special attention to the judgment of business logic.
Basically that's it. NULL looks simple, but it is easy to get stuck when querying, counting, and connecting tables, especially when used with other conditions. Just remember one thing: NULL is not equal to any value, including itself, and it can almost avoid most of the problems.
The above is the detailed content of Handling NULL Values in MySQL Columns and Queries. For more information, please follow other related articles on the PHP Chinese website!
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