Dynamic SQL: Building Flexible and Powerful Queries
Jul 28, 2025 am 01:25 AMDynamic SQL is a SQL statement that is dynamically built during the operation of the program, which is used to implement flexible query logic. It splices SQL according to runtime conditions and is suitable for scenarios such as multi-condition query, dynamic sorting, batch operations, etc. When using it, SQL injection should be prevented through parameterized queries, whitelist verification, query constructors, etc. 1. Multi-condition query should determine whether the parameters exist and dynamically add WHERE conditions; 2. Dynamic sorting requires verification of column names and restricting paging parameters; 3. Batch operations should control the length of the parameters and use transactions. In addition, looping splicing, generating complex statements should be avoided, and execution plans should be checked using EXPLAIN to optimize performance. Rational use of dynamic SQL can improve flexibility, but requires both security and efficiency.
Dynamic SQL is a very practical tool in database development, especially in scenarios where different query logics need to be generated according to runtime conditions. It allows you to splice SQL statements according to different parameters, thereby achieving more flexible and powerful data operation capabilities.

What is dynamic SQL?
Simply put, dynamic SQL is to build SQL statements during the program's running, rather than writing them in code or stored procedures. For example, after you enter the search box in the front end, the background will combine different query statements based on different filter conditions (such as name, age, and region).
The benefits of this approach are obvious: it is highly flexible and adapted to various user inputs; the disadvantage is that if handled improperly, it can easily cause SQL injection risks or make statements difficult to maintain.

How to safely splice SQL statements?
The most common method of splicing SQL is string splicing. But be aware:
- Avoid direct splicing of user input, parameterized query should be used
- Do whitelist verification of field names and table names
- Use the query constructor provided by the framework (such as Laravel's Query Builder, MyBatis'
<if></if>
tag, etc.)
For example, in PHP, if you write this way:

$sql = "SELECT * FROM users WHERE name = '" . $_GET['name'] . "'";
Then your system will be easily attacked. It should be changed to a preprocessing statement using PDO or MySQLi:
$stmt = $pdo->prepare("SELECT * FROM users WHERE name = ?"); $stmt->execute([$_GET['name']]);
This way, even if the user enters malicious content, it will not affect the database structure.
Common dynamic query scenarios and processing methods
1. Multi-condition query
This is one of the most common dynamic SQL scenarios. For example, a user management page can be filtered by username, email, role and other conditions.
At this time, you can use a method similar to the following:
- Determine whether each parameter exists
- If it exists, add the corresponding
WHERE
clause - Pay attention to the first condition and do not add extra
AND
For example in Java MyBatis:
<select id="findUsers" resultType="User"> SELECT * FROM users <where> <if test="name != null"> AND name = #{name} </if> <if test="email != null"> AND email = #{email} </if> </where> </select>
2. Dynamic sorting and pagination
Sometimes you need to let the user choose which field to sort by, or set how many records to display per page. Although this type of operation is simple, it is easy to ignore security.
suggestion:
- The sorting sequence names need to be checked in whitelist and cannot be spliced directly
- The maximum value should be limited to the pagination offset and quantity to prevent resource exhaustion
3. Batch operation
For example, deleting multiple users at once, or inserting multiple records. At this time, you can use dynamically to generate IN()
clause or batch insertion statements.
Notice:
- There is a limit on parameter length (especially URL requests to pass parameters)
- Consider transaction control to avoid data inconsistency caused by partial execution failure
Performance optimization and precautions
Although dynamic SQL is powerful, improper use can also cause performance problems:
- Avoid splicing of SQL in loops, which will cause repeated parsing of statements, affecting database cache efficiency
- Don't generate too complex statements to increase maintenance costs
- Use EXPLAIN to check execution plans and make sure to go to indexes
In addition, although some ORM tools encapsulate dynamic query functions, the underlying layer is still spliced SQL. So even if you use a high-level framework, you have to understand the principles behind it.
Overall, dynamic SQL is a very useful trick, especially when building a general query interface or complex business system. As long as you pay attention to safety and performance and use it reasonably, you can maximize its value. Basically that's it.
The above is the detailed content of Dynamic SQL: Building Flexible and Powerful Queries. For more information, please follow other related articles on the PHP Chinese website!

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