Summary: in this tutorial, you will learn how to use MySQL cursor in stored procedures to iterate through a result set returned by a SELECT statement.
Introduction to MySQL cursor
To handle a result set inside a stored procedure, you use a cursor. A cursor allows you to iterate a set of rows returned by a query and process each row individually.
MySQL cursor is read-only, non-scrollable and asensitive.
- Read-only: you cannot update data in the underlying table through the cursor.
- Non-scrollable: you can only fetch rows in the order determined by the
SELECTstatement. You cannot fetch rows in the reversed order. In addition, you cannot skip rows or jump to a specific row in the result set. - Asensitive: there are two kinds of cursors: asensitive cursor and insensitive cursor. An asensitive cursor points to the actual data, whereas an insensitive cursor uses a temporary copy of the data. An asensitive cursor performs faster than an insensitive cursor because it does not have to make a temporary copy of data. However, any change that made to the data from other connections will affect the data that is being used by an asensitive cursor, therefore, it is safer if you do not update the data that is being used by an asensitive cursor. MySQL cursor is asensitive.
You can use MySQL cursors in stored procedures, stored functions, and triggers.
Working with MySQL cursor
First, declare a cursor by using the DECLARE statement:
DECLARE cursor_name CURSOR FOR SELECT_statement;Code language: SQL (Structured Query Language) (sql)The cursor declaration must be after any variable declaration. If you declare a cursor before the variable declarations, MySQL will issue an error. A cursor must always associate with a SELECT statement.
Next, open the cursor by using the OPEN statement. The OPEN statement initializes the result set for the cursor, therefore, you must call the OPEN statement before fetching rows from the result set.
Code language: SQL (Structured Query Language) (sql)OPEN cursor_name;
Then, use the FETCH statement to retrieve the next row pointed by the cursor and move the cursor to the next row in the result set.
Code language: SQL (Structured Query Language) (sql)FETCH cursor_name INTO variables list;
After that, check if there is any row available before fetching it.
Finally, deactivate the cursor and release the memory associated with it using the CLOSE statement:
Code language: SQL (Structured Query Language) (sql)CLOSE cursor_name;
It is a good practice to always close a cursor when it is no longer used.
When working with MySQL cursor, you must also declare a NOT FOUND handler to handle the situation when the cursor could not find any row.
Because each time you call the FETCH statement, the cursor attempts to read the next row in the result set. When the cursor reaches the end of the result set, it will not be able to get the data, and a condition is raised. The handler is used to handle this condition.
To declare a NOT FOUND handler, you use the following syntax:
DECLARE CONTINUE HANDLER FOR NOT FOUND SET finished = 1;Code language: SQL (Structured Query Language) (sql)The finished is a variable to indicate that the cursor has reached the end of the result set. Notice that the handler declaration must appear after variable and cursor declaration inside the stored procedures.
The following diagram illustrates how MySQL cursor works.
MySQL Cursor Example
We’ll develop a stored procedure that creates an email list of all employees in the employees table in the sample database.
First, declare some variables, a cursor for looping over the emails of employees, and a NOT FOUND handler:
DECLARE finished INTEGER DEFAULT 0;
DECLARE emailAddress varchar(100) DEFAULT "";
-- declare cursor for employee email
DEClARE curEmail
CURSOR FOR
SELECT email FROM employees;
-- declare NOT FOUND handler
DECLARE CONTINUE HANDLER
FOR NOT FOUND SET finished = 1;
Code language: SQL (Structured Query Language) (sql)Next, open the cursor by using the OPEN statement:
Code language: SQL (Structured Query Language) (sql)OPEN curEmail;
Then, iterate the email list, and concatenate all emails where each email is separated by a semicolon(;):
getEmail: LOOP
FETCH curEmail INTO emailAddress;
IF finished = 1 THEN
LEAVE getEmail;
END IF;
-- build email list
SET emailList = CONCAT(emailAddress,";",emailList);
END LOOP getEmail;Code language: SQL (Structured Query Language) (sql)After that, inside the loop, we used the finished variable to check if there is an email in the list to terminate the loop.
Finally, close the cursor using the CLOSE statement:
Code language: SQL (Structured Query Language) (sql)CLOSE email_cursor;
The createEmailList stored procedure is as follows:
DELIMITER $$
CREATE PROCEDURE createEmailList (
INOUT emailList varchar(4000)
)
BEGIN
DECLARE finished INTEGER DEFAULT 0;
DECLARE emailAddress varchar(100) DEFAULT "";
-- declare cursor for employee email
DEClARE curEmail
CURSOR FOR
SELECT email FROM employees;
-- declare NOT FOUND handler
DECLARE CONTINUE HANDLER
FOR NOT FOUND SET finished = 1;
OPEN curEmail;
getEmail: LOOP
FETCH curEmail INTO emailAddress;
IF finished = 1 THEN
LEAVE getEmail;
END IF;
-- build email list
SET emailList = CONCAT(emailAddress,";",emailList);
END LOOP getEmail;
CLOSE curEmail;
END$$
DELIMITER ;Code language: SQL (Structured Query Language) (sql)You can test the createEmailList stored procedure using the following script:
SET @emailList = "";
CALL createEmailList(@emailList);
SELECT @emailList;Code language: SQL (Structured Query Language) (sql)In this tutorial, we have shown you how to use MySQL cursor to iterate a result set and process each row accordingly.
