Control statements form the heart of most languages since they control the execution of other sets of statements. These are found in SQL too, and should be exploited for uses such as query filtering and query optimization through careful selection of tuples that match our requirement. In this post, we explore the Case-Switch statement in SQL.
The CASE statement is SQL’s way of handling if/then logic.
Syntax:
There can be two valid ways of going about the case-switch statements.

1. 1. The first takes a variable called case_value and matches it with some statement_list.

      CASE case_value
          WHEN when_value THEN statement_list
          [WHEN when_value THEN statement_list] ...
          [ELSE statement_list]
      END CASE
      

   2. The second considers a search_condition instead of variable equality and executes the statement_list accordingly.

      CASE
          WHEN search_condition THEN statement_list
          [WHEN search_condition THEN statement_list] ...
          [ELSE statement_list]
      END CASE
      

Examples:

Say we have a relation, Faculty.

Faculty Table:

Let’s say we would like to modify this table such that if the department name is ‘CS’, it gets modified to ‘Computer Science’, if it is ‘EC’ it gets modified to ‘Electronics and Communication’, and if it is ‘HSS’ it gets modified to ‘Humanities and Social Sciences’. This can be achieved using case statement.

Sample Query:
Consider a variable, department_name which is entered in the SQL code.

CASE department_name
 WHEN 'CS'
  THEN UPDATE Faculty SET
  department='Computer Science';
 WHEN 'EC'
  THEN UPDATE Faculty SET
  department='Electronics and Communication';
 ELSE UPDATE Faculty SET
 department='Humanities and Social Sciences';
END CASE

Output:

The department name corresponding to the given input gets renamed.

Consider another query which selects all the fields corresponding to the Faculty table. Since the values written in the Gender field are single character values (M/F), we would like to present them in a more readable format.

SELECT FacultyID, Name, Department,
CASE Gender
 WHEN'M' THEN 'Male'
 WHEN'F' THEN 'Female'
END
FROM Faculty

Output:

Consider yet another application of case-switch in SQL- custom sorting.

CREATE PROCEDURE GetFaculty(@ColToSort varchar(150)) AS
SELECT FacultyID, Name, Gender, Department
FROM Customers
ORDER BY
  CASE WHEN @ColToSort='Department' THEN Department
       WHEN @ColToSort='Name' THEN Name
       WHEN @ColToSort='Gender' THEN Gender
       ElSE FacultyID
  END 

Output:

The output gets sorted according to the provided field.

The above procedure (function) takes a variable of varchar data type as its argument, and on the basis of that, sorts the tuples in the Faculty table.