Each table should have a primary key, because the primary key not only ensures record uniqueness and non-emptyness, but also serves as a clustered index to improve query performance. 1. The primary key forces non-empty to avoid data ambiguity; 2. The primary key serves as the basis for physical storage order to improve search and connection efficiency; 3. No primary key may lead to duplicate data, misoperation and foreign key failure. When selecting a primary key, you can use auto-increment ID, UUID or stable natural keys, but frequent modifications should be avoided. The primary key is different from a unique index. The former emphasizes entity integrity, while the latter is used for business verification and allows null values. When setting primary keys, you need to be named uniformly, use compound primary keys with caution, avoid frequent updates, and select data types reasonably.

Primary keys are essential in SQL tables to maintain data integrity. They not only ensure the uniqueness of each record, but also provide the infrastructure for database design. If you are building or maintaining a database, understanding how to use primary keys correctly is the key to avoiding data confusion and query errors.

Why should each table have a primary key

You may ask: "If I can guarantee uniqueness with a unique index, why do I still need to use the primary key?" In fact, the primary key is not just a constraint on uniqueness, it also implies the requirement of non-empty (NULL). This means that each record must have a clear identification and there will be no "cannot find who it is".

In addition, primary keys usually automatically become clustered indexes, which determine the physical storage order of data on disk. This is very obvious for tables that frequently search and join operations.

Problems that occur in tables without primary keys include:

• Increased risk of inserting duplicate data
• It is easy to operate incorrectly when updating or deleting specific rows
• Foreign key associations become unreliable or even impossible to establish
• Query efficiency may decline, especially on large tables

How to choose the right field as primary key

The primary key can be a naturally existing business field (such as ID number and order number), or it can be a artificially generated proxy key (such as self-increasing ID). Which method to choose depends on your specific needs.

Common practices include:

• Use INT or BIGINT type autoincrement columns as primary keys, which are suitable for most scenarios.
• If you need a globally unique identity, you can choose UUID or GUID , but be careful about its impact on index performance.
• Natural primary keys are suitable for stable and unambiguous fields, such as country codes, ISBNs, etc.

It should be noted that once the primary key is selected, try not to change it easily. Because many foreign keys or other logic may depend on it, the modification cost is higher.

The difference between primary key and unique index

Although both primary keys and unique indexes can guarantee the uniqueness of field values, the two are not equivalent.

The primary key emphasizes entity integrity, while the unique index is more used for business-level uniqueness verification. The primary key is not allowed to be empty, while the unique index allows multiple NULL values (depending on the database implementation).

For example:
A user table can use the "ID number" as the unique index to prevent duplicate registration, but the primary key is still the self-increased user ID. This not only ensures the stability within the system, but also meets business rules.

Best practices for setting primary keys

Setting the primary key is not simply adding a constraint, but the following aspects need to be considered:

• Unified naming specifications : It is recommended to use consistent naming rules, such as PK_ , for easy subsequent maintenance.
• Composite primary keys should be used with caution : unless multiple fields do require a record to be identified together, composite primary keys are not recommended because it increases query and maintenance complexity.
• Avoid frequent update of primary key values : Once the primary key value is referenced by other tables, updates will bring chain reactions and should be avoided as much as possible.
• Pay attention to the data type and length of the primary key : select the appropriate data type based on the data volume estimate to avoid the difficulty of later expansion.

Basically that's it. The primary key seems simple, but it has a profound impact in practical applications. If you are not careful, you will pose hidden dangers.

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