How to use if nested statements in MySQL stored procedures

1. Introduction to if statement

The if statement is a branching structure statement that performs different operations based on conditions. The if statement usually consists of a conditional expression and one or more statements. The prerequisite for executing the statements in the if statement is that the value of the conditional expression is true, otherwise the if statement block will be skipped.

The syntax of the if statement is as follows:

if(condition)then
    statement;
else
    statement;
end if;

Among them, condition is the conditional expression, and statement is the SQL statement that needs to be executed.

2. Introduction to nested if statements

A nested if statement is within an if statement block, with one or more if statement blocks nested inside. The purpose is to perform different operations based on different conditions. The syntax of if nested statements is as follows:

if(condition1)then
    statement;
    if(condition2)then
        statement;
    else
        statement;
    end if;
else if(condition3)then
    statement;
else
    statement;
end if;

Among them, condition1 is the conditional expression of the first level if; condition2 is the conditional expression of the second level if; condition3 is the conditional expression of the first else if ;statement is the SQL statement that needs to be executed.

3. Example of if nested statement

The following is an example of a stored procedure using if nested statement:

delimiter //
create procedure test_if_nested(
    in student_name varchar(50),
    out result_msg varchar(50)
)
begin
    declare student_score int;
    select score into student_score from student where name = student_name;
    if(student_score >= 90)then
        set result_msg = '优秀';
        if(student_score = 100)then
            set result_msg = concat(result_msg, '，满分');
        end if;
    else if(student_score >= 60)then
        set result_msg = '及格';
    else
        set result_msg = '不及格';
    end if;
end //
delimiter ;

This stored procedure is used Judge the student's performance based on their scores:

1. If the score is greater than or equal to 90 points, it is considered excellent; if it is 100 points, "full score" is added;

2. If the score is greater than or equal to 60 points, it is a pass;

3. If the score is less than 60 points, it is a fail.

4. Stored procedure call

The stored procedure can be called through the call command. The syntax is as follows:

call procedure_name(argument1, argument2, ...);

Among them, procedure_name is the name of the stored procedure, argument1, argument2, etc. are the parameters of the stored procedure.

For example, to call the stored procedure above, you can use the following command:

call test_if_nested('张三', @result_msg);
select @result_msg as result;

Pass in a student name parameter and output the result through the out parameter. The result is as follows:

+-------------+
| result      |
+-------------+
| 及格       |
+-------------+

Through the above calling method, we can obtain the student's score based on his name and judge the student's grade based on the score.

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