Generated columns in MySQL 5.7 automatically derive values from other columns, improving efficiency and reducing application logic. 1. They come in two types: virtual (computed on read) and stored (computed on write). 2. Define them using GENERATED ALWAYS AS with an expression, specifying VIRTUAL or STORED. 3. Use virtual for space savings and stored for indexing or performance. 4. Avoid inserting values manually unless changing sql_mode. 5. Expressions must be deterministic and reference only base columns. Proper use ensures clean schema design and optimized query performance.
If you're working with MySQL 5.7 or newer and want to keep your database structure clean and efficient, generated columns can be a great tool. They let you store values derived from other columns automatically — no application logic needed.
Here’s how to use them effectively without getting lost in the details.
What Are Generated Columns?
Generated columns are virtual (or stored) columns whose values are based on expressions involving other columns in the same table. For example, if you have first_name and last_name, you can create a generated column that combines them into full_name.
There are two types:
- Virtual: Not stored on disk; computed when read.
- Stored: Physically saved to disk; computed when written.
This feature is especially useful for simplifying queries or avoiding redundant application-level calculations.
How to Define a Generated Column
When creating or altering a table, use the GENERATED ALWAYS AS clause followed by the expression. Here's a basic example:
CREATE TABLE employees (
id INT PRIMARY KEY,
first_name VARCHAR(50),
last_name VARCHAR(50),
full_name VARCHAR(100) GENERATED ALWAYS AS (CONCAT(first_name, ' ', last_name)) STORED
);A few things to note:
- The data type must be explicitly declared.
- Expressions can include functions and operators, but not subqueries.
- Use
VIRTUALinstead ofSTOREDif you don’t need physical storage.
Also, be careful with case sensitivity and whitespace — especially when using string concatenation or date formatting.
When to Use Virtual vs Stored
Choosing between virtual and stored depends on your use case.
Use virtual columns when:
- You rarely query the generated value.
- You want to save disk space.
- Your query engine supports virtual column optimization (like in views or joins).
Use stored columns when:
- You need to index the generated value.
- Performance matters during reads.
- You want to export or replicate the data as-is.
Keep in mind that indexing a virtual column is possible in some versions, but it requires the column to be persisted in certain contexts, like InnoDB.
Common Gotchas
Working with generated columns isn't always smooth sailing. Here are a few common issues:
- You can’t manually insert values into a generated column unless you set the session variable
SET sql_mode = 'NO_VALUE_FOR_GENERATED_COLUMNS';. - Expressions can’t reference other generated columns directly — they can only use base columns.
- If your expression uses a function, make sure it's deterministic, or MySQL might reject it.
- Be cautious with data types — if your expression returns more characters than the defined length, truncation happens silently (unless strict mode is on).
For example, this will fail unless you allow overrides:
INSERT INTO employees (id, first_name, last_name, full_name) VALUES (1, 'John', 'Doe', 'Jane Smith');
MySQL will throw an error because full_name is generated.
That’s the core of using generated columns in MySQL 5.7 . It’s a powerful feature once you understand its limitations and best practices.
The above is the detailed content of Using generated columns in MySQL 5.7 and higher. For more information, please follow other related articles on the PHP Chinese website!
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