Use the fetch statement to obtain the repetition of the last row of cursor data

• Problem description ：《MySQL Will know 》 Medium P179 After the execution of the stored procedure, a row is repeatedly inserted in the new table .

• stored procedure processorders（ To calculate the orders The total amount of each order in the table and deposit in a new table ）

# Create a stored procedure to calculate the total amount of each order and store it in a new table 
DELIMITER //
CREATE PROCEDURE processorders()
BEGIN
	# Declare local variables 
	DECLARE done BOOLEAN DEFAULT 0;
	DECLARE o INT;
	DECLARE t DECIMAL(8,2);
	
	# declare cursor 
	DECLARE ordernumbers CURSOR
	FOR
	SELECT order_num FROM orders;
	
	# Declare exception handling , When sql Status as 02000 when , take done Set to 1
	DECLARE CONTINUE HANDLER FOR SQLSTATE '02000' SET done = 1;
	
	# Create a new table to store the results 
	CREATE TABLE IF NOT EXISTS ordertotals
	(order_num INT,
	total DECIMAL(8,2)
	
	);
	
	# Open cursor 
	OPEN ordernumbers;
	
	# Insert all rows in a loop 
	REPEAT
		# Get the order number , When the intra row pointer points to the next row of the last row, executing this statement will make SQLSTATE='02000'
		# Cause the execution of the upper handle processing to make done = 1
		FETCH ordernumbers INTO o;
		
		# Calculate the total amount with tax , The result will be returned to the parameter t, The stored procedure is at the bottom 
		CALL ordertotal(o,1,t);
		
		# Insert the order number and total amount into the table 
		INSERT INTO ordertotals(order_num,total)
		VALUES(o,t);
	UNTIL done # The end cycle condition is done For the wrong 0
	END REPEAT;
	
	# Close cursor 
	CLOSE ordernumbers;
END//

# Execute query statement 
CALL processorders()//

• The result

• Problem analysis ： When the pointer in the cursor points to the next row of the last row , Re execution

  FETCH ordernumbers INTO o;

  When this statement , To take the current line and move the pointer down one line , Because the position pointed by the pointer is illegal , So there is a row error , Cause the upper handle processing to trigger , bring done = 1, And then o No new data is inserted , Or the old data of the last round （ That is, the order number in the last line ）, Therefore, the last line is repeatedly inserted twice .

• resolvent ： stay FETCH Judge immediately after the statement is executed done Is it modified to 1, If it is modified to 1, Then the following insert code will not be executed . The stored procedure code above is modified to

DELIMITER //
CREATE PROCEDURE processorders()
BEGIN
	# Declare local variables 
	DECLARE done BOOLEAN DEFAULT 0;
	DECLARE o INT;
	DECLARE t DECIMAL(8,2);
	
	# declare cursor 
	DECLARE ordernumbers CURSOR
	FOR
	SELECT order_num FROM orders;
	
	# Declare exception handling , When sql Status as 02000 when , take done Set to 1
	DECLARE CONTINUE HANDLER FOR SQLSTATE '02000' SET done = 1;
	
	# Create a new table to store the results 
	CREATE TABLE IF NOT EXISTS ordertotals
	(order_num INT,
	total DECIMAL(8,2)
	
	);
	
	# Open cursor 
	OPEN ordernumbers;
	
	# Insert all rows in a loop 
	REPEAT
		# Get the order number , When the intra row pointer points to the next row of the last row, executing this statement will make SQLSTATE='02000'
		# Cause the execution of the upper handle processing to make done = 1
		FETCH ordernumbers INTO o;
		IF !done THEN # The key position of the correction 
		
		# Calculate the total amount with tax , The result will be returned to the parameter t, The stored procedure is at the bottom 
		CALL ordertotal(o,1,t);
		
		# Insert the order number and total amount into the table 
		INSERT INTO ordertotals(order_num,total)
		VALUES(o,t);
		END IF;
	UNTIL done 
	END REPEAT;
	
	# Close cursor 
	CLOSE ordernumbers;
END//

Query again ordertotals The result of the table is

Reference material ：

1. https://stackoverflow.com/questions/16900482/mysql-cursor-fetching-same-result-twice
2. https://my.oschina.net/u/1383934/blog/667444  《mysql The last row of the cursor is repeated 》