How can I simulate an array variable in MySQL?

问题

It appears that MySQL doesn\'t have array variables. What should I use instead?

---

There seem to be two alternatives suggested: A set-type scalar and temporary tables. The question I linked to suggests the former. But is it good practice to use these instead of array variables? Alternatively, if I go with sets, what would be the set-based idiom equivalent to foreach?

回答1:

Well, I've been using temporary tables instead of array variables. Not the greatest solution, but it works.

Note that you don't need to formally define their fields, just create them using a SELECT:

CREATE TEMPORARY TABLE IF NOT EXISTS my_temp_table
SELECT first_name FROM people WHERE last_name = 'Smith';

(See also Create temporary table from select statement without using Create Table.)

回答2:

You can achieve this in MySQL using WHILE loop:

SET @myArrayOfValue = '2,5,2,23,6,';

WHILE (LOCATE(',', @myArrayOfValue) > 0)
DO
    SET @value = ELT(1, @myArrayOfValue);
    SET @myArrayOfValue= SUBSTRING(@myArrayOfValue, LOCATE(',',@myArrayOfValue) + 1);

    INSERT INTO `EXEMPLE` VALUES(@value, 'hello');
END WHILE;

EDIT: Alternatively you can do it using UNION ALL:

INSERT INTO `EXEMPLE`
(
 `value`, `message`
)
(
 SELECT 2 AS `value`, 'hello' AS `message`
 UNION ALL
 SELECT 5 AS `value`, 'hello' AS `message`
 UNION ALL
 SELECT 2 AS `value`, 'hello' AS `message`
 UNION ALL
 ...
);

回答3:

Try using FIND_IN_SET() function of MySql e.g.

SET @c = 'xxx,yyy,zzz';

SELECT * from countries 
WHERE FIND_IN_SET(countryname,@c);

Note: You don't have to SET variable in StoredProcedure if you are passing parameter with CSV values.

回答4:

Dont know about the arrays, but there is a way to store comma-separated lists in normal VARCHAR column.

And when you need to find something in that list you can use the FIND_IN_SET() function.

回答5:

Nowadays using a JSON array would be an obvious answer.

Since this is an old but still relevant question I produced a short example. JSON functions are available since mySQL 5.7.x / MariaDB 10.2.3

I prefer this solution over ELT() because it's really more like an array and this 'array' can be reused in the code.

But be careful: It (JSON) is certainly much slower than using a temporary table. Its just more handy. imo.

Here is how to use a JSON array:

SET @myjson = '["gmail.com","mail.ru","arcor.de","gmx.de","t-online.de",
                "web.de","googlemail.com","freenet.de","yahoo.de","gmx.net",
                "me.com","bluewin.ch","hotmail.com","hotmail.de","live.de",
                "icloud.com","hotmail.co.uk","yahoo.co.jp","yandex.ru"]';

SELECT JSON_LENGTH(@myjson);
-- result: 19

SELECT JSON_VALUE(@myjson, '$[0]');
-- result: gmail.com

And here a little example to show how it works in a function/procedure:

DELIMITER //
CREATE OR REPLACE FUNCTION example() RETURNS varchar(1000) DETERMINISTIC
BEGIN
  DECLARE _result varchar(1000) DEFAULT '';
  DECLARE _counter INT DEFAULT 0;
  DECLARE _value varchar(50);

  SET @myjson = '["gmail.com","mail.ru","arcor.de","gmx.de","t-online.de",
                "web.de","googlemail.com","freenet.de","yahoo.de","gmx.net",
                "me.com","bluewin.ch","hotmail.com","hotmail.de","live.de",
                "icloud.com","hotmail.co.uk","yahoo.co.jp","yandex.ru"]';

  WHILE _counter < JSON_LENGTH(@myjson) DO
    -- do whatever, e.g. add-up strings...
    SET _result = CONCAT(_result, _counter, '-', JSON_VALUE(@myjson, CONCAT('$[',_counter,']')), '#');

    SET _counter = _counter + 1;
  END WHILE;

  RETURN _result;
END //
DELIMITER ;

SELECT example();

回答6:

DELIMITER $$
CREATE DEFINER=`mysqldb`@`%` PROCEDURE `abc`()
BEGIN
  BEGIN 
    set @value :='11,2,3,1,'; 
    WHILE (LOCATE(',', @value) > 0) DO
      SET @V_DESIGNATION = SUBSTRING(@value,1, LOCATE(',',@value)-1); 
      SET @value = SUBSTRING(@value, LOCATE(',',@value) + 1); 
      select @V_DESIGNATION;
    END WHILE;
  END;
END$$
DELIMITER ;

回答7:

Maybe create a temporary memory table with columns (key, value) if you want associative arrays. Having a memory table is the closest thing to having arrays in mysql

回答8:

Here’s how I did it.

First, I created a function that checks whether a Long/Integer/whatever value is in a list of values separated by commas:

CREATE DEFINER = 'root'@'localhost' FUNCTION `is_id_in_ids`(
        `strIDs` VARCHAR(255),
        `_id` BIGINT
    )
    RETURNS BIT(1)
    NOT DETERMINISTIC
    CONTAINS SQL
    SQL SECURITY DEFINER
    COMMENT ''
BEGIN

  DECLARE strLen    INT DEFAULT 0;
  DECLARE subStrLen INT DEFAULT 0;
  DECLARE subs      VARCHAR(255);

  IF strIDs IS NULL THEN
    SET strIDs = '';
  END IF;

  do_this:
    LOOP
      SET strLen = LENGTH(strIDs);
      SET subs = SUBSTRING_INDEX(strIDs, ',', 1);

      if ( CAST(subs AS UNSIGNED) = _id ) THEN
        -- founded
        return(1);
      END IF;

      SET subStrLen = LENGTH(SUBSTRING_INDEX(strIDs, ',', 1));
      SET strIDs = MID(strIDs, subStrLen+2, strLen);

      IF strIDs = NULL or trim(strIds) = '' THEN
        LEAVE do_this;
      END IF;

  END LOOP do_this;

   -- not founded
  return(0);

END;

So now you can search for an ID in a comma-separated list of IDs, like this:

select `is_id_in_ids`('1001,1002,1003',1002);

And you can use this function inside a WHERE clause, like this:

SELECT * FROM table1 WHERE `is_id_in_ids`('1001,1002,1003',table1_id);

This was the only way I found to pass an "array" parameter to a PROCEDURE.

回答9:

I know that this is a bit of a late response, but I recently had to solve a similar problem and thought that this may be useful to others.

Background

Consider the table below called 'mytable':

The problem was to keep only latest 3 records and delete any older records whose systemid=1 (there could be many other records in the table with other systemid values)

It would be good is you could do this simply using the statement

DELETE FROM mytable WHERE id IN (SELECT id FROM `mytable` WHERE systemid=1 ORDER BY id DESC LIMIT 3)

However this is not yet supported in MySQL and if you try this then you will get an error like

...doesn't yet support 'LIMIT & IN/ALL/SOME subquery'

So a workaround is needed whereby an array of values is passed to the IN selector using variable. However, as variables need to be single values, I would need to simulate an array. The trick is to create the array as a comma separated list of values (string) and assign this to the variable as follows

SET @myvar := (SELECT GROUP_CONCAT(id SEPARATOR ',') AS myval FROM (SELECT * FROM `mytable` WHERE systemid=1 ORDER BY id DESC LIMIT 3 ) A GROUP BY A.systemid);

The result stored in @myvar is

5,6,7

Next, the FIND_IN_SET selector is used to select from the simulated array

SELECT * FROM mytable WHERE FIND_IN_SET(id,@myvar);

The combined final result is as follows:

SET @myvar := (SELECT GROUP_CONCAT(id SEPARATOR ',') AS myval FROM (SELECT * FROM `mytable` WHERE systemid=1 ORDER BY id DESC LIMIT 3 ) A GROUP BY A.systemid);
DELETE FROM mytable WHERE FIND_IN_SET(id,@myvar);

I am aware that this is a very specific case. However it can be modified to suit just about any other case where a variable needs to store an array of values.

I hope that this helps.

回答10:

Isn't the point of arrays to be efficient? If you're just iterating through values, I think a cursor on a temporary (or permanent) table makes more sense than seeking commas, no? Also cleaner. Lookup "mysql DECLARE CURSOR".

For random access a temporary table with numerically indexed primary key. Unfortunately the fastest access you'll get is a hash table, not true random access.

回答11:

This works fine for list of values:

SET @myArrayOfValue = '2,5,2,23,6,';

WHILE (LOCATE(',', @myArrayOfValue) > 0)
DO
SET @value = ELT(1, @myArrayOfValue);
    SET @STR = SUBSTRING(@myArrayOfValue, 1, LOCATE(',',@myArrayOfValue)-1);
    SET @myArrayOfValue = SUBSTRING(@myArrayOfValue, LOCATE(',', @myArrayOfValue) + 1);

    INSERT INTO `Demo` VALUES(@STR, 'hello');
END WHILE;

回答12:

Both versions using sets didn't work for me (tested with MySQL 5.5). The function ELT() returns the whole set. Considering the WHILE statement is only avaible in PROCEDURE context i added it to my solution:

DROP PROCEDURE IF EXISTS __main__;

DELIMITER $
CREATE PROCEDURE __main__()
BEGIN
    SET @myArrayOfValue = '2,5,2,23,6,';

    WHILE (LOCATE(',', @myArrayOfValue) > 0)
    DO
        SET @value = LEFT(@myArrayOfValue, LOCATE(',',@myArrayOfValue) - 1);    
        SET @myArrayOfValue = SUBSTRING(@myArrayOfValue, LOCATE(',',@myArrayOfValue) + 1);
    END WHILE;
END;
$
DELIMITER ;

CALL __main__;

To be honest, i don't think this is a good practice. Even if its realy necessary, this is barely readable and quite slow.

回答13:

I'm surprised none of the answers mention ELT/FIELD.

ELT/FIELD works very similar to an array especially if you have static data.

FIND_IN_SET also works similar but doesn't have a built in complementary function but it's easy enough to write one.

mysql> select elt(2,'AA','BB','CC');
+-----------------------+
| elt(2,'AA','BB','CC') |
+-----------------------+
| BB                    |
+-----------------------+
1 row in set (0.00 sec)

mysql> select field('BB','AA','BB','CC');
+----------------------------+
| field('BB','AA','BB','CC') |
+----------------------------+
|                          2 |
+----------------------------+
1 row in set (0.00 sec)

mysql> select find_in_set('CC','AA,BB,CC');
+------------------------------+
| find_in_set('BB','AA,BB,CC') |
+------------------------------+
|                            2 |
+------------------------------+
1 row in set (0.00 sec)

mysql>  SELECT SUBSTRING_INDEX(SUBSTRING_INDEX('AA,BB,CC',',',2),',',-1);
+-----------------------------------------------------------+
| SUBSTRING_INDEX(SUBSTRING_INDEX('AA,BB,CC',',',2),',',-1) |
+-----------------------------------------------------------+
| BB                                                        |
+-----------------------------------------------------------+
1 row in set (0.01 sec)

回答14:

Another way to see the same problem. Hope helpfull

DELIMITER $$
CREATE PROCEDURE ARR(v_value VARCHAR(100))
BEGIN

DECLARE v_tam VARCHAR(100);
DECLARE v_pos VARCHAR(100);

CREATE TEMPORARY TABLE IF NOT EXISTS split (split VARCHAR(50));

SET v_tam = (SELECT (LENGTH(v_value) - LENGTH(REPLACE(v_value,',',''))));
SET v_pos = 1;

WHILE (v_tam >= v_pos)
DO
    INSERT INTO split 
    SELECT SUBSTRING_INDEX(SUBSTRING_INDEX(v_value,',',v_pos),',', -1);
    SET v_pos = v_pos + 1;
END WHILE;

SELECT * FROM split;

DROP TEMPORARY TABLE split;

END$$


CALL ARR('1006212,1006404,1003404,1006505,444,');

回答15:

In MYSQL version after 5.7.x, you can use JSON type to store an array. You can get value of an array by a key via MYSQL.

回答16:

Inspired by the function ELT(index number, string1, string2, string3,…),I think the following example works as an array example:

set @i := 1;
while @i <= 3
do
  insert into table(val) values (ELT(@i ,'val1','val2','val3'...));
set @i = @i + 1;
end while;

Hope it help.

回答17:

I Think I can improve on this answer. Try this:

The parameter 'Pranks' is a CSV. ie. '1,2,3,4.....etc'

CREATE PROCEDURE AddRanks(
IN Pranks TEXT
)
BEGIN
  DECLARE VCounter INTEGER;
  DECLARE VStringToAdd VARCHAR(50);
  SET VCounter = 0;
  START TRANSACTION;
  REPEAT
    SET VStringToAdd = (SELECT TRIM(SUBSTRING_INDEX(Pranks, ',', 1)));
    SET Pranks = (SELECT RIGHT(Pranks, TRIM(LENGTH(Pranks) - LENGTH(SUBSTRING_INDEX(Pranks, ',', 1))-1)));
    INSERT INTO tbl_rank_names(rank)
    VALUES(VStringToAdd);
    SET VCounter = VCounter + 1;
  UNTIL (Pranks = '')
  END REPEAT;
  SELECT VCounter AS 'Records added';
  COMMIT;
END;

This method makes the searched string of CSV values progressively shorter with each iteration of the loop, which I believe would be better for optimization.

回答18:

I would try something like this for multiple collections. I'm a MySQL beginner. Sorry about the function names, couldn't decide on what names would be best.

delimiter //

drop  procedure init_
//
create procedure init_()
begin
  CREATE TEMPORARY TABLE if not exists 
    val_store(  
    realm  varchar(30) 
    ,  id  varchar(30) 
    ,  val   varchar(255) 
    ,  primary key ( realm , id )
    );
end;
//

drop function if exists get_
//
create function get_( p_realm varchar(30) , p_id varchar(30) )
  returns varchar(255)
  reads sql data
begin 
  declare ret_val varchar(255);
  declare continue handler for 1146 set ret_val = null;
  select val into ret_val from val_store where id = p_id;
  return ret_val;
end;
//

drop procedure if exists set_
//
create procedure set_( p_realm varchar(30) , p_id varchar(30) , p_val varchar(255) )
begin
  call init_(); 
  insert into val_store (realm,id,val) values (p_realm , p_id , p_val) on duplicate key update val = p_val;
end;
//

drop   procedure if exists remove_
//
create procedure remove_( p_realm varchar(30) , p_id varchar(30) )
begin
  call init_();
  delete from val_store where realm = p_realm and id = p_id;
end;
//

drop   procedure if exists erase_
//
create procedure erase_( p_realm varchar(30) ) 
begin
  call init_();
  delete from val_store where realm = p_realm;
end;
//

call set_('my_array_table_name','my_key','my_value');

select get_('my_array_table_name','my_key');

回答19:

Have you tried using PHP's serialize()? That allows you to store the contents of a variable's array in a string PHP understands and is safe for the database (assuming you've escaped it first).

$array = array(
    1 => 'some data',
    2 => 'some more'
);

//Assuming you're already connected to the database
$sql = sprintf("INSERT INTO `yourTable` (`rowID`, `rowContent`) VALUES (NULL, '%s')"
     ,  serialize(mysql_real_escape_string($array, $dbConnection)));
mysql_query($sql, $dbConnection) or die(mysql_error());

You can also do the exact same without a numbered array

$array2 = array(
    'something' => 'something else'
);

or

$array3 = array(
    'somethingNew'
);

来源：https://stackoverflow.com/questions/12176709/how-can-i-simulate-an-array-variable-in-mysql