SQL CASE statements are used to implement conditional logic, mainly divided into two forms: simple CASE and search CASE. Simple CASE compares expressions with multiple values, while search CASE supports conditional expressions, which are more flexible. For example, use search CASE in student ratings: SELECT name, CASE WHEN score >= 90 THEN 'A' WHEN score >= 80 THEN 'B' ELSE 'C' END AS grade FROM students; in addition, CASE can not only be used for SELECT to generate new fields, but also for WHERE, ORDER BY and JOIN conditions such as dynamic sorting and data cleaning. When using it, you need to pay attention to the order of the condition affecting the results. It is recommended to add alias to avoid excessive nesting, while keeping the format clear to improve maintainability. There are usually no problems with performance, but complex conditions should be used with caution when using large amounts of data.

SQL CASE statements are an important tool for implementing conditional logic, and are especially useful in queries that need to return different results according to different situations. It is a bit like the if-else structure in a programming language, but the syntax is more concise and more suitable for writing in SQL queries.

Basic structure: The two writing methods must be distinguished

There are two common forms of CASE statements: simple CASE and search CASE .

• A simple CASE is to compare an expression with multiple possible values:

  

   CASE column_name
    WHEN value1 THEN result1
    WHEN value2 THEN result2
    ELSE default_result
  END

• Searching CASE is more flexible and supports the use of conditional expressions:

   CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    ELSE default_result
  END

For example: If you want to rate students based on your score, it is more appropriate to use search CASE:

 SELECT name,
       CASE
         WHEN score >= 90 THEN 'A'
         WHEN score >= 80 THEN 'B'
         ELSE 'C'
       END AS grade
FROM students;

Practical application scenario: Don't just use it to display data

Many people only use CASE in SELECT to generate new fields, but in fact it can also be used in WHERE, ORDER BY, and even JOIN conditions.

For example, if you want to sort dynamically, you will give priority to displaying high-rated users, which can be used in ORDER BY:

 SELECT name, rating
FROM users
ORDER BY CASE
           WHEN rating > 4 THEN 0
           ELSE 1
         END, rating DESC;

This allows the ones with the highest ratings to be ranked first, and the rest can be arranged in descending order of ratings.

Another typical scenario is to clean data, such as replacing null or outliers with default values:

 SELECT product_name,
       CASE
         WHEN price < 0 THEN 0
         ELSE price
       END AS adjusted_price
FROM products;

Notes on using: both order and readability are important

There are several details that are easily overlooked:

• The order will affect the results : especially when searching for CASE, once a certain WHEN condition is met, the subsequent one will not be judged.
• Remember to add END alias : Although it is not a mandatory requirement, for the convenience of subsequent quotations, it is best to give an alias to the CASE result.
• Avoid too deep nesting : If there are many layers of nesting CASE, it is recommended to split it into views or temporary tables to improve maintainability.
• Performance is generally OK , but frequent use of multiple complex conditions in big data scales may slow down the query speed.

In addition, don't format and layout too casually. Although SQL does not strictly require indentation, a clear format makes it easier for you to see logical relationships, especially during debugging.

Basically that's it. CASE does not seem complicated, but if you want to use it well, you have to practice more practical examples, especially some boundary situations and multi-condition combinations.

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