The syntax for adding columns in different database systems varies greatly, and varies from database to database. For example: MySQL: ALTER TABLE users ADD COLUMN email VARCHAR(255); PostgreSQL: ALTER TABLE users ADD COLUMN email VARCHAR(255) NOT NULL UNIQUE;Oracle: ALTER TABLE users ADD email VARCHAR2(255);SQL Server: ALTER TABLE users ADD email VARCH

Syntax for adding columns in different database systems? Of course it is different! This question is so wonderful, it is simply a soul-question of the compulsory course in the introductory database. If all databases are the same, wouldn’t the database engineer be unemployed? No matter how much jokes are, this involves the design concept, underlying implementation, and even historical reasons of the database.

Let’s talk about the conclusion first: It’s definitely different! Each database system has its own dialect, just like so many languages ??in the world, you can't expect to communicate smoothly with a person who only speaks French. MySQL, PostgreSQL, Oracle, SQL Server, the syntax difference between adding columns is huge!

For example, you want to add a column named email to a table named users , the type is VARCHAR(255) .

MySQL:

 <code class="sql">ALTER TABLE users ADD COLUMN email VARCHAR(255);</code>

It is concise and clear, you can understand it at a glance. This is the style of MySQL, simple and crude, and efficient.

PostgreSQL:

 <code class="sql">ALTER TABLE users ADD COLUMN email VARCHAR(255);</code>

Why? Exactly the same as MySQL? Don't be too happy too early, this is just a superficial phenomenon. The power of PostgreSQL is that it supports a variety of data types and constraints, so you can add more tricks here, such as:

 <code class="sql">ALTER TABLE users ADD COLUMN email VARCHAR(255) NOT NULL UNIQUE;</code>

This sentence SQL not only adds columns, but also adds NOT NULL and UNIQUE constraints to ensure that the mailbox is not empty and unique. Of course, MySQL can do it, but the writing may be slightly different.

Oracle:

 <code class="sql">ALTER TABLE users ADD email VARCHAR2(255);</code>

Oracle likes to use VARCHAR2 instead of VARCHAR , which is its usual style and you have to remember.

SQL Server:

 <code class="sql">ALTER TABLE users ADD email VARCHAR(255);</code>

The syntax of SQL Server looks very similar to that of MySQL and PostgreSQL, but there are still differences in details. For example, their choices may be different when dealing with large text types.

After seeing this, you should understand, right? It looks similar on the surface, but in actual operation, you have to check the corresponding documents based on the database system you use. Don’t be lazy, documents are your good friends!

To go deeper, different databases also support data types differently, such as JSON type, some databases are natively supported, and some databases need to be extended. So, when adding columns, you need to consider not only the syntax, but also whether the data type is compatible and the version of the database.

In addition, there is another pitfall, which is the concurrency problem. If you add columns in a high concurrency environment, various unexpected problems may arise. At this time, you need to consider the locking mechanism or use transactions to ensure data consistency. Don't underestimate these details, they will cause your hair to fall off handfuls in the middle of the night when you debug it.

In short, remember one sentence: There is no general column addition syntax! Only by choosing the correct syntax and understanding the characteristics of the database can you write efficient and reliable code. Only by practicing more and trampling more pitfalls can you become a real database expert!

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