The processing of if-then-else logic in SQL is mainly implemented through CASE expressions. 1. CASE is divided into simple CASE and search CASE, which is more flexible and can be used for complex condition judgments; 2. It is recommended to use ELSE to avoid returning NULL, and can be applied to SELECT, WHERE, and ORDER BY clauses; 3. Some databases support IF functions, which are only applicable to two-choice judgments; 4. Dynamic filtering logic can be implemented through boolean expressions in WHERE clauses; 5. Pay attention to the use of indexes when optimizing performance.

Handling if-then-else logic in SQL is not as intuitive as writing programs, but it is not difficult. SQL itself does not have a special if-else statement, but similar effects can be achieved through some functions and expressions. The most commonly used method is the CASE expression, which can simulate the logical flow of if-then-else very well.

Implement if-then-else using CASE expressions

CASE is the most common and flexible way to judge conditions in SQL. It has two ways of writing: simple CASE and search CASE .

Simple CASE : Applicable to cases where fields are equal to certain values. For example:

 SELECT name,
       CASE department
           WHEN 'HR' THEN 'Human Resources'
           WHEN 'IT' THEN 'Information Technology'
           ELSE 'Other'
       END AS department_name
FROM employees;

Search CASE : More suitable for handling complex conditional judgments, such as range judgment:

 SELECT name, salary,
       CASE
           WHEN salary < 3000 THEN &#39;Low&#39;
           WHEN salary BETWEEN 3000 AND 6000 THEN &#39;Medium&#39;
           ELSE &#39;High&#39;
       END AS salary_level
FROM employees;

• It is recommended to use search CASE first, because it is more flexible and not limited to cases where fields are equal to a certain value.
• Pay attention to the use of ELSE , which can avoid returning NULL when there is no match.
• CASE can be used in multiple clauses such as SELECT, WHERE, ORDER BY, etc.

Use IF functions (some databases support)

Some databases (such as MySQL) provide IF() functions that can simplify simple judgment logic. The syntax is as follows:

 SELECT name, IF(salary > 5000, 'High', 'Low') AS salary_level
FROM employees;

• This function is only suitable for simple judgments of choosing one of two.
• Note that not all databases support IF() , such as PostgreSQL and SQL Server do not.
• If you are not sure whether the database supports it, it is recommended to use CASE uniformly.

Implement conditional filtering in WHERE clause

Sometimes we want to dynamically filter data based on a certain condition, such as "If a certain parameter is A, check condition X, otherwise check condition Y". At this time, it can be implemented using CASE or Boolean logic.

For example, you want to filter data based on user roles:

 SELECT *
FROM orders
WHERE (CASE
           WHEN user_role = 'admin' THEN order_status IN ('pending', 'completed')
           ELSE order_status = 'pending'
       END);

Or use a boolean expression more concisely:

 SELECT *
FROM orders
WHERE (user_role = 'admin' AND order_status IN ('pending', 'completed'))
   OR (user_role != 'admin' AND order_status = 'pending');

• This type of writing is common in dynamic SQL or report queries.
• Pay attention to the order of logic and the use of brackets to avoid incorrect matches.
• If there are requirements for query performance, it is recommended to add a suitable index.

Let's summarize

The most common and recommended way to deal with if-then-else in SQL is to use CASE expressions. It has clear syntax and a wide range of application. If the database supports it, you can also use the IF() function for simple judgment. When making conditional judgments in the WHERE clause, flexible query logic can be written in combination with Boolean expressions.

Basically all is it, not complicated but it is easy to ignore details.

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