The primary key of mysql can be null

The MySQL primary key cannot be empty because the primary key is a key attribute that uniquely identifies each row in the database. If the primary key can be empty, the record cannot be uniquely identifies, which will lead to data confusion. When using self-incremental integer columns or UUIDs as primary keys, you should consider factors such as efficiency and space occupancy and choose an appropriate solution.

Can the MySQL primary key be empty? The answer is: No!

You might ask, why? The main key thing, the most core existence in the database, cannot be empty? Doesn’t this limit my design freedom? Don't worry, let me make a good fight for you.

To put it bluntly, database design is to build a home for data, so that the data can be lived in comfort, convenient to find, and safe and reliable. The primary key is the house number of this house. Every house must have it and it must be unique. Imagine if the house number can be empty, then isn’t this community a mess? You are looking for someone but can’t find the house number, how can you find it? The same is true for databases. If the primary key is empty, you cannot uniquely identify a record. It's like you give each file a name, but you allow some files to have no names. Then when you want to find a file, wouldn't you have to search through the entire hard drive?

Therefore, the primary key of MySQL is not allowed to be empty, which is a fundamental constraint of the database and the cornerstone of a relational database. If you insist on making it empty, the database engine will directly report an error to you and ask you to correct it back obediently.

Some might say, what if I design a table that allows some records to have no primary key value for the time being? In this case, you might consider using other alternatives, such as using a self-growing integer column as the primary key, or using a UUID as the primary key. The self-growing primary key is simple and direct, and has high efficiency, but it has a disadvantage, that is, once the record is inserted, the primary key value is fixed and difficult to modify. Although UUID can ensure global uniqueness, it is relatively long, takes up a lot of space, and has relatively low query efficiency. Which option to choose depends on your actual needs.

Let's look at some code examples to feel the power of the primary key and the cost of making mistakes:

The correct way to do it:

 <code class="sql">CREATE TABLE users ( id INT AUTO_INCREMENT PRIMARY KEY, -- 自增主键，最常见的方案username VARCHAR(255) NOT NULL, email VARCHAR(255) UNIQUE );</code>

This code creates a table named users with id column as the primary key and is self-incremented. AUTO_INCREMENT ensures that each newly inserted record will get a unique id value and does not require us to specify it manually. The NOT NULL constraint ensures that id column cannot be empty. username and email columns are also subject to corresponding constraints to ensure data integrity.

Wrong way: (try to make the primary key empty)

 <code class="sql">CREATE TABLE users_wrong ( id INT PRIMARY KEY, -- 这里没有NOT NULL约束，试图让主键为空username VARCHAR(255), email VARCHAR(255) ); INSERT INTO users_wrong (username, email) VALUES ('testuser', 'test@example.com'); -- 这条语句可以执行，因为没有对主键进行赋值INSERT INTO users_wrong (id, username, email) VALUES (NULL, 'anotheruser', 'another@example.com'); -- 这条语句会报错，因为主键不允许为空</code>

This code tries to create a table where the primary key can be empty, but the database throws an error when you try to insert a record with the primary key empty. This once again proves the iron law that the primary key cannot be empty.

Deeper thinking:

There is still a lot to discuss about the choice of primary keys. For example, compound primary keys (multiple columns form primary keys) can improve data query efficiency in some scenarios, but they are also more complex to design and require careful trade-offs. In addition, the selection of primary key type is also very important. INT type is relatively common, but for hyper-large databases, you may need to consider using BIGINT type. These details need to be selected according to actual conditions, and there is no absolute best solution.

Remember that primary key is the cornerstone of the database. Only by understanding its importance and following standard design principles can we build a stable and reliable database system. Don't try to challenge the rules of the database, or you will pay the price.

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