mysql cursor with example

8/10/2019 MySQL Cursor With Example

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MySQL Cursor

Summary: in this tutorial, you will learn how to useMySQL

cursorin stored procedures to iterate througha 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 accordingly.

MySQL cursor is read only, non-scrollable and asensitive.

Read only: you cannot update data in the underlying

table through the cursor.

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Non-scrollable: you can only fetch rows in the order

determined by the SELECT statement. 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 dont 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, you have to declare a cursor by using the DECLARE

statement:

The cursor declaration must be after any variable declaration. If

you declare a cursor before variables declaration, MySQL will

issue an error. A cursor must always be associated with a

SELECT statement.

Next, you 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.

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1 DECLAREcursor_nameCURSORFORSELECT_statement;

1 OPENcursor_name;
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Then, you 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.

After that, you can check to see if there is any row available

before fetching it.

Finally, you call the CLOSE statement to deactivate the cursor

and release the memory associated with it as follows:

When the cursor is no longer used, you should close it.

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 isused to handle this condition.

To declare a NOT FOUND handler, you use the following

syntax:

Where finished is a variable to indicate that the cursor hasreached 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.

1 FETCHcursor_nameINTOvariableslist;

1 CLOSEcursor_name;

1 DECLARECONTINUEHANDLERFORNOTFOUNDSETfinished= 1;


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MySQL Cursor Example

We are going to develop a stored procedure that builds an emaillistof all employees in the employees table in the MySQL

sample database.

First, we declare some variables, a cursor for looping over the

emails of employees, and a NOT FOUND handler:

Next, we open the email_cursor by using the OPEN

statement:

Then, we iterate the email list, and concatenate all emails where

each email is separated by a semicolon(;):

After that, inside the loop we used the v_finished variable to

checkif there is any email in the list to terminate the loop.

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DECLAREfinishedINTEGERDEFAULT0;

DECLAREemailvarchar(255)DEFAULT"";

-- declare cursor for employee email

DEClAREemail_cursorCURSORFOR

SELECTemailFROMemployees;

-- declare NOT FOUND handler

DECLARECONTINUEHANDLER

FORNOTFOUNDSETfinished= 1;

1 OPENemail_cursor;

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get_email: LOOP

FETCHemail_cursorINTOv_email;IFv_finished= 1THEN

LEAVEget_email;

ENDIF;

-- build email list

SETemail_list= CONCAT(v_email,";",email_list);

ENDLOOPget_email;
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Finally, we close the cursor using the CLOSE statement:

The build_email_list stored procedure is as follows:

You can test the build_email_list stored procedure using

the following script:

In this tutorial, we have shown you how to use MySQL cursor to

1 CLOSEemail_cursor;

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DELIMITER$$

CREATEPROCEDUREbuild_email_list (INOUTemail_listvarchar(

BEGIN

DECLAREv_finishedINTEGERDEFAULT0;

DECLAREv_emailvarchar(100)DEFAULT"";

-- declare cursor for employee email

DEClAREemail_cursorCURSORFOR

SELECTemailFROMemployees;

-- declare NOT FOUND handlerDECLARECONTINUEHANDLER

FORNOTFOUNDSETv_finished= 1;

OPENemail_cursor;

get_email: LOOP

FETCHemail_cursorINTOv_email;

IFv_finished= 1THEN

LEAVEget_email;

ENDIF;

-- build email list

SETemail_list= CONCAT(v_email,";",email_list);

ENDLOOPget_email;

CLOSEemail_cursor;

END$$

DELIMITER;

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SET@email_list= "";

CALLbuild_email_list(@email_list);

SELECT@email_list;


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iterate a result set and process each row accordingly.

Related Tutorials

Getting Started with MySQL Stored Procedures

MySQL Stored Procedure Variables

Introduction to MySQL Stored Procedures

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mysql cursor with example

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