MySQL cursors

• 1. Overview of cursors
• 2. Steps to use the cursor
• • 2.1 Declaring Cursors
  • 2.2 Open Cursor
  • 2.3 Using a cursor (getting data from a cursor)
  • 2.4 Close the cursor
• 3. Case presentation

1. Overview of cursors

In the sql statement we wrote before, although the returned records can be limited by filter conditions, we have no way to locate each record in the result set, like a pointer, forward positioning, backward positioning, or randomLocate a record.To solve this problem, we can use cursors.
The cursor allows us to locate each record in the result set and operate on the data in the pointed record

• In SQL, a cursor is a temporary database object
• The cursor acts as a pointer
• We can manipulate the data by manipulating the cursor
• Cursors in MySQL can be used in stored procedures and functions

2. Steps to use the cursor

• The cursor must be declared before the handler is declared, and variables and conditions must also be declared before the cursor or handler is declared
• Using a cursor generally goes through the following four steps:
• Declare a cursor
• Open cursor
• Using a cursor
• Close the cursor

Let's take a look at how to use it.

2.1 Declaring Cursors

• In MySQL, use the declare keyword to declare a cursor

• Syntax

  DECLARE cursor name CURSOR FOR query statement;

• The select statement here is used to return a result set that creates a cursor

• cursor in English means cursor, cursor

• Simple memory means using a query statement to describe the cursor

• Example

  DECLARE cur_emp CURSOR FOR
  SELECT employee_id,salary FROM employees;

2.2 open cursor

• After defining the cursor, if you want to use it, you must open the cursor

• When the cursor is opened, the query result set of the select statement will be sent to the cursor work area to prepare for the subsequent cursor to read the records in the result set one by one

• Syntax

  open cursor name

2.3 Using a cursor (getting data from a cursor)

• Syntax

  fetch cursorname into var_name…

• We use a cursor to read the current row, we can save the data to a variable, and the cursor pointer points to the next row

• If the data read by the cursor has multiple columns, we can assign it to multiple variables after into

• var_name should be defined before the cursor declaration

• The number of fields in the result set of the cursor query must be the same as the number of variables after into, otherwise an error will be reported when the stored procedure is executed.

2.4 close cursor

• Syntax: close cursor name
• Because using a cursor will occupy system resources, it needs to be closed in time. If it is not closed in time, the cursor will be saved until the end of the stored procedure, which affects the efficiency of system operation. Closing the cursor can release the system resources occupied by the cursor.
• After closing the cursor, we can no longer retrieve the data rows in the query result. If we need to retrieve, we need to open the cursor again.

3. Case presentation

• Create stored procedure "get_count_by_limit_total_salary()"

• Declare IN parameter limit_total_salary, DOUBLE type; declare OUT parameter total_count, INT type.

• The function of the function can realize the accumulation of the salary values ​​of the employees with the highest salary until the total salary reaches the value of the limit_total_salary parameter, and returns the accumulated number to total_count.

  DELIMITER //

  CREATE PROCEDURE get_count_by_limit_total_salary(IN limit_total_salary DOUBLE,OUT total_count INT)

  BEGIN
  DECLARE sum_salary DOUBLE DEFAULT 0; #Record accumulatedTotal salary
  DECLARE cursor_salary DOUBLE DEFAULT 0; #Record a salary value
  DECLARE emp_count INT DEFAULT 0; #Record the number of loops
  #Define cursor
  DECLARE emp_cursor CURSOR FOR SELECT salary FROM employees ORDER BYsalary DESC;
  #Open the cursor
  OPEN emp_cursor;

  REPEAT#Use the cursor (get data from the cursor)FETCH emp_cursor INTO cursor_salary;SET sum_salary = sum_salary + cursor_salary;SET emp_count = emp_count + 1;UNTIL sum_salary >= limit_total_salaryEND REPEAT;SET total_count = emp_count;#Close the cursorCLOSE emp_cursor;

  END //

  DELIMITER ;

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