Database
Mysql Tutorial
What are Common Table Expressions (CTEs) and how to use the WITH clause?
What are Common Table Expressions (CTEs) and how to use the WITH clause?
Jun 20, 2025 am 01:02 AM
CTE (Common Table Expression) is a way in SQL for defining temporary result sets. It is defined by the WITH keyword and exists only during the current query execution. Its core role is to simplify complex query structures and improve readability and maintenance. The main uses of CTE include: 1. Simplify nested queries to make multi-layer logic clear and separate; 2. Support recursive queries, suitable for processing hierarchical or tree-like data structures; 3. Replace views, providing temporary logical abstraction without changing the database structure. When using it, you should pay attention to: the scope of action of CTE is limited to the queries that follow. Multiple CTEs can be defined and naming conflicts can be avoided. The performance is comparable to subqueries and does not guarantee improvement in execution efficiency. When choosing a CTE or a temporary table, it should be judged based on the usage scenario. CTE is more suitable for intermediate results processing in a single query.
CTEs (Common Table Expressions) is a very practical way to define temporary result sets in SQL, which is often used to simplify complex queries. They are defined by the WITH keyword, exist within the execution scope of a single query statement, and can be referenced multiple times.
What is CTE?
CTE is a named temporary result set that only exists during the execution of your current query. You can understand it as a "query in a query", but it's easier to read and maintain than a subquery.
The basic structure is as follows:
WITH cte_name AS (
SELECT ...
)
SELECT * FROM cte_name;The biggest benefit of CTE is to make complex queries clearer. For example, when you need multi-layer nested subqueries, using CTE can disassemble and write each layer, making the logic clearer.
Common uses of CTE
1. Simplify nested queries
Using CTE can significantly improve readability when your SQL query contains multiple subqueries.
For example: You want to find out the information of the highest paid employees in each department. If you do not use CTE, you may write three-layer nested subqueries; and if you use CTE, you can first calculate the maximum salary for each department and then make a connection.
WITH MaxSalaries AS (
SELECT department_id, MAX(salary) AS max_salary
FROM employees
GROUP BY department_id
)
SELECT e.*
FROM employees e
JOIN MaxSalaries ms
ON e.department_id = ms.department_id AND e.salary = ms.max_salary;This writing not only has a clear structure, but also facilitates debugging and subsequent modifications.
2. Recursive query
CTE also supports recursive queries, which are very suitable for processing tree or hierarchical data, such as organizational structures, directory structures, etc.
The syntax is as follows:
WITH RECURSIVE cte_name AS (
-- Initial query SELECT ...
UNION ALL
-- Recursive part SELECT ...
)
SELECT * FROM cte_name;For example, finding all subordinates of an employee (including indirect subordinates) can be achieved using recursive CTE.
3. Alternative View (View)
If you don't want to create a permanent view, but want to abstract and reuse a certain piece of logic, you can use CTE instead. CTE is temporary and does not affect the database structure.
Notes on using CTE
- The scope of action is limited to the query that follows : CTE is not saved to the database, but can only be used in
SELECT,INSERT,UPDATE, orDELETEimmediately after it. - There can be multiple CTEs : You can define multiple CTEs in a
WITHclause, separated by commas.
For example:
WITH cte1 AS (...),
cte2 AS (...)
SELECT ...- Naming conflicts should be avoided : If you write multiple CTEs in a query, the names cannot be repeated.
- Performance is not necessarily better : although CTE makes the code clearer, it will essentially be processed by the optimizer as a subquery and does not necessarily improve performance. Therefore, in the case of large data volume, you should pay attention to the execution plan.
Tips: How to choose CTE and temporary tables?
If you only need to use intermediate results in one query, it is easier to use CTE;
If you want to use this intermediate result multiple times throughout the session, or the amount of data is particularly large, then consider using a temporary table ( TEMPORARY TABLE ).
Basically that's it. CTE is a very practical feature in modern SQL, especially suitable for building clear structure and easy to maintain query statements. Once you master it, you will find it easier to write complex queries.
The above is the detailed content of What are Common Table Expressions (CTEs) and how to use the WITH clause?. For more information, please follow other related articles on the PHP Chinese website!
Hot AI Tools
Undress AI Tool
Undress images for free
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Clothoff.io
AI clothes remover
Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!
Hot Article
Hot Tools
Notepad++7.3.1
Easy-to-use and free code editor
SublimeText3 Chinese version
Chinese version, very easy to use
Zend Studio 13.0.1
Powerful PHP integrated development environment
Dreamweaver CS6
Visual web development tools
SublimeText3 Mac version
God-level code editing software (SublimeText3)
Hot Topics
What is GTID (Global Transaction Identifier) and what are its advantages?
Jun 19, 2025 am 01:03 AM
GTID (Global Transaction Identifier) ??solves the complexity of replication and failover in MySQL databases by assigning a unique identity to each transaction. 1. It simplifies replication management, automatically handles log files and locations, allowing slave servers to request transactions based on the last executed GTID. 2. Ensure consistency across servers, ensure that each transaction is applied only once on each server, and avoid data inconsistency. 3. Improve troubleshooting efficiency. GTID includes server UUID and serial number, which is convenient for tracking transaction flow and accurately locate problems. These three core advantages make MySQL replication more robust and easy to manage, significantly improving system reliability and data integrity.
What is a typical process for MySQL master failover?
Jun 19, 2025 am 01:06 AM
MySQL main library failover mainly includes four steps. 1. Fault detection: Regularly check the main library process, connection status and simple query to determine whether it is downtime, set up a retry mechanism to avoid misjudgment, and can use tools such as MHA, Orchestrator or Keepalived to assist in detection; 2. Select the new main library: select the most suitable slave library to replace it according to the data synchronization progress (Seconds_Behind_Master), binlog data integrity, network delay and load conditions, and perform data compensation or manual intervention if necessary; 3. Switch topology: Point other slave libraries to the new master library, execute RESETMASTER or enable GTID, update the VIP, DNS or proxy configuration to
How to connect to a MySQL database using the command line?
Jun 19, 2025 am 01:05 AM
The steps to connect to the MySQL database are as follows: 1. Use the basic command format mysql-u username-p-h host address to connect, enter the username and password to log in; 2. If you need to directly enter the specified database, you can add the database name after the command, such as mysql-uroot-pmyproject; 3. If the port is not the default 3306, you need to add the -P parameter to specify the port number, such as mysql-uroot-p-h192.168.1.100-P3307; In addition, if you encounter a password error, you can re-enter it. If the connection fails, check the network, firewall or permission settings. If the client is missing, you can install mysql-client on Linux through the package manager. Master these commands
Why is InnoDB the recommended storage engine now?
Jun 17, 2025 am 09:18 AM
InnoDB is MySQL's default storage engine because it outperforms other engines such as MyISAM in terms of reliability, concurrency performance and crash recovery. 1. It supports transaction processing, follows ACID principles, ensures data integrity, and is suitable for key data scenarios such as financial records or user accounts; 2. It adopts row-level locks instead of table-level locks to improve performance and throughput in high concurrent write environments; 3. It has a crash recovery mechanism and automatic repair function, and supports foreign key constraints to ensure data consistency and reference integrity, and prevent isolated records and data inconsistencies.
Why do indexes improve MySQL query speed?
Jun 19, 2025 am 01:05 AM
IndexesinMySQLimprovequeryspeedbyenablingfasterdataretrieval.1.Theyreducedatascanned,allowingMySQLtoquicklylocaterelevantrowsinWHEREorORDERBYclauses,especiallyimportantforlargeorfrequentlyqueriedtables.2.Theyspeedupjoinsandsorting,makingJOINoperation
How to add the MySQL bin directory to the system PATH
Jul 01, 2025 am 01:39 AM
To add MySQL's bin directory to the system PATH, it needs to be configured according to the different operating systems. 1. Windows system: Find the bin folder in the MySQL installation directory (the default path is usually C:\ProgramFiles\MySQL\MySQLServerX.X\bin), right-click "This Computer" → "Properties" → "Advanced System Settings" → "Environment Variables", select Path in "System Variables" and edit it, add the MySQLbin path, save it and restart the command prompt and enter mysql--version verification; 2.macOS and Linux systems: Bash users edit ~/.bashrc or ~/.bash_
What are the transaction isolation levels in MySQL, and which is the default?
Jun 23, 2025 pm 03:05 PM
MySQL's default transaction isolation level is RepeatableRead, which prevents dirty reads and non-repeatable reads through MVCC and gap locks, and avoids phantom reading in most cases; other major levels include read uncommitted (ReadUncommitted), allowing dirty reads but the fastest performance, 1. Read Committed (ReadCommitted) ensures that the submitted data is read but may encounter non-repeatable reads and phantom readings, 2. RepeatableRead default level ensures that multiple reads within the transaction are consistent, 3. Serialization (Serializable) the highest level, prevents other transactions from modifying data through locks, ensuring data integrity but sacrificing performance;
What are the ACID properties of a MySQL transaction?
Jun 20, 2025 am 01:06 AM
MySQL transactions follow ACID characteristics to ensure the reliability and consistency of database transactions. First, atomicity ensures that transactions are executed as an indivisible whole, either all succeed or all fail to roll back. For example, withdrawals and deposits must be completed or not occur at the same time in the transfer operation; second, consistency ensures that transactions transition the database from one valid state to another, and maintains the correct data logic through mechanisms such as constraints and triggers; third, isolation controls the visibility of multiple transactions when concurrent execution, prevents dirty reading, non-repeatable reading and fantasy reading. MySQL supports ReadUncommitted and ReadCommi.
