sql with 递归
Introduction
SQL is a standard programming language used for managing relational databases. One of the most interesting and powerful features of SQL is the ability to perform recursive queries. Recursive queries allow SQL to operate on hierarchical data structures and are useful in many applications such as organizing file systems or tracking ancestry. In this article, we will explore the basics of recursive SQL queries and how they can be implemented using the 'WITH' statement and the common table expression (CTE).
The WITH Statement
The WITH statement is used to CREATE a temporary table that can be referenced in subsequent SQL queries. The temporary table is called a common table expression (CTE) and is defined within the WITH statement. The CTE can then be used as if it were a real table in the rest of the SQL query. This is useful for many operations, but it is especially useful for recursive SQL queries because it allows us to define a recursive table in a concise and elegant manner.
Recursive Queries and Hierarchical Data Structures
Recursive SQL queries are used to operate on hierarchical data structures, such as trees or graphs. The basic idea behind a recursive query is to define a table that references itself. For example, if we have a table of employees, we might want to create a recursive query that returns all of the employees that report to a given manager, as well as all of their subordinates. To do this, we can define a CTE that references itself to create a hierarchical data structure that represents the employee hierarchy. We can then query this structure to get a list of all employees that work for a given manager.
Example: Recursive Query for Calculating Factorials
To illustrate the power of recursive SQL queries, let's take a look at a simple example: calculating factorials. A factorial is defined as the product of all positive integers up to a given number. For example, 5! = 5 * 4 * 3 * 2 * 1 = 120. We can use a recursive SQL query to calculate factorials as follows:
```SQLWITH RECURSIVE factorials(num, fact) AS ( VALUES (1, 1) UNION ALL SELECT num + 1, (num + 1) * fact FROM factorials WHERE num < 10)SELECT num, fact FROM factorials;```
In this example, we define a CTE called 'factorials' that has two columns: 'num' and 'fact'. We start with the values (1, 1) and then recursively generate the next row by incrementing 'num' and multiplying it by 'fact'. We stop when 'num' reaches 10. Finally, we select the num and fact columns from the 'factorials' CTE to display the results. This will output a table of the first 10 factorials.
Conclusion
Recursive SQL queries are a powerful feature of SQL that allow us to work with hierarchical data structures. Using the WITH statement and common table expressions, we can define recursive tables in a concise and elegant manner. Recursive queries can be used for a wide range of applications, from organizing file systems to calculating factorials. By understanding how to use recursive SQL queries, we can improve our ability to work with relational data and create more sophisticated applications.
