如何在 MySQL 中模拟数组变量？

Question

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 ?

Answer 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:

DROP TEMPORARY TABLE IF EXISTS my_temp_table;
CREATE TEMPORARY TABLE my_temp_table
    SELECT first_name FROM people WHERE last_name = 'Smith';

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

Answer 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
 ...
);

Answer 3

Try using FIND_IN_SET() function of MySql eg

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.

Answer 4

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();

Answer 5

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.

Answer 6

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 if 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.

Answer 7

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 ;

Answer 8

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

Answer 9

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.

Answer 10

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('BB','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)

Answer 11

Is an array variable really necessary?

I ask because I originally landed here wanting to add an array as a MySQL table variable. I was relatively new to database design and trying to think of how I'd do it in a typical programming language fashion.

But databases are different. I thought I wanted an array as a variable, but it turns out that's just not a common MySQL database practice.

Standard Practice

The alternative solution to arrays is to add an additional table, and then reference your original table with a foreign key.

As an example, let's imagine an application that keeps track of all the items every person in a household wants to buy at the store.

The commands for creating the table I originally envisioned would have looked something like this:

#doesn't work
CREATE TABLE Person(
  name VARCHAR(50) PRIMARY KEY
  buy_list ARRAY
);

I think I envisioned buy_list to be a comma-separated string of items or something like that.

But MySQL doesn't have an array type field, so I really needed something like this:

CREATE TABLE Person(
  name VARCHAR(50) PRIMARY KEY
);
CREATE TABLE BuyList(
  person VARCHAR(50),
  item VARCHAR(50),
  PRIMARY KEY (person, item),
  CONSTRAINT fk_person FOREIGN KEY (person) REFERENCES Person(name)
);

Here we define a constraint named fk_person. It says that the 'person' field in BuyList is a foreign key. In other words, it's a primary key in another table, specifically the 'name' field in the Person table, which is what REFERENCES denotes.

We also defined the combination of person and item to be the primary key, but technically that's not necessary.

Finally, if you want to get all the items on a person's list, you can run this query:

SELECT item FROM BuyList WHERE person='John';

This gives you all the items on John's list. No arrays necessary!

Answer 12

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;

Answer 13

This is my solution to use a variable containing a list of elements. You can use it in simple queries (no need to use store procedures or create tables).

I found somewhere else on the site the trick to use the JSON_TABLE function (it works in mysql 8, I dunno of it works in other versions).

set @x = '1,2,3,4' ;

select c.NAME
from colors c
where 
  c.COD in ( 
    select * 
    from json_table( 
      concat('[',@x,']'),
      '$[*]' columns (id int path '$') ) t ) ;

Also, you may need to manage the case of one or more variables set to empty_string . In this case I added another trick (the query does not return error even if x , y , or both x and y are empty strings):

set @x = '' ;
set @y = 'yellow' ;

select c.NAME
from colors 
where 
  if(@y = '', 1 = 1, c.NAME = @y)
  and if(@x = '', 1, c.COD) in ( 
    select * 
    from json_table(
      concat('[',if(@x = '', 1, @x),']'),
      '$[*]' columns (id int path '$') ) t) ;

Answer 14

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.

Answer 15

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.

Answer 16

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,');

Answer 17

If we have one table like that

mysql> select * from user_mail;
+------------+-------+
| email      | user | 
+------------+-------+-
| email1@gmail |     1 | 
| email2@gmail |     2 |
+------------+-------+--------+------------+

and the array table:

mysql> select * from user_mail_array;
+------------+-------+-------------+
| email      | user | preferences |
+------------+-------+-------------+
| email1@gmail |     1 |           1 |
| email1@gmail |     1 |           2 |
| email1@gmail |     1 |           3 |
| email1@gmail |     1 |           4 |
| email2@gmail |     2 |           5 |
| email2@gmail |     2 |           6 |

We can select the rows of the second table as one array with CONCAT function:

mysql> SELECT t1.*, GROUP_CONCAT(t2.preferences) AS preferences
     FROM user_mail t1,user_mail_array t2
       where t1.email=t2.email and t1.user=t2.user
     GROUP BY t1.email,t1.user;

+------------+-------+--------+------------+-------------+
| email      | user | preferences |
+------------+-------+--------+------------+-------------+
|email1@gmail |     1 | 1,3,2,4     |
|email2@gmail |     2 | 5,6         |
+------------+-------+--------+------------+-------------+

Answer 18

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.

Answer 19

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.

Answer 20

Here is an example for MySQL for looping through a comma delimited string.

DECLARE v_delimited_string_access_index INT;
DECLARE v_delimited_string_access_value VARCHAR(255);
DECLARE v_can_still_find_values_in_delimited_string BOOLEAN;

SET v_can_still_find_values_in_delimited_string = true;
SET v_delimited_string_access_index = 0;
WHILE (v_can_still_find_values_in_delimited_string) DO
  SET v_delimited_string_access_value = get_from_delimiter_split_string(in_array, ',', v_delimited_string_access_index); -- get value from string
  SET v_delimited_string_access_index = v_delimited_string_access_index + 1;
  IF (v_delimited_string_access_value = '') THEN
    SET v_can_still_find_values_in_delimited_string = false; -- no value at this index, stop looping
  ELSE
    -- DO WHAT YOU WANT WITH v_delimited_string_access_value HERE
  END IF;
END WHILE;

this uses the get_from_delimiter_split_string function defined here: https://stackoverflow.com/a/59666211/3068233

Answer 21

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.

Answer 22

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');

Answer 23

Rather than Saving data as a array or in one row only you should be making diffrent rows for every value received. This will make it much simpler to understand rather than putting all together.

Answer 24

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'
);