Mysql truncates concatenated result of a GROUP_CON

问题:

I've created a view which uses GROUP_CONCAT to concatenate results from a query on products column with data type of 'varchar(7) utf8_general_ci' in a column named concat_products. The problem is that mysql truncates value of concat_products column. phpMyAdmin says the data type of concat_products column is varchar(341) utf8_bin

table products:

CREATE TABLE `products`(
`productId` tinyint(2) unsigned NOT NULL AUTO_INCREMENT, 
`product` varchar(7) COLLATE utf8_general_ci NOT NULL, 
`price` mediumint(5) unsigned NOT NULL, 
PRIMARY KEY (`productId`)) 
ENGINE=InnoDB AUTO_INCREMENT=28 DEFAULT CHARSET=utf8 COLLATE=utf8_general_ci

concat_products_vw View:

CREATE VIEW concat_products_vw AS
SELECT
  `userId`,
    GROUP_CONCAT(CONCAT_WS('_', `product`, `productId`, `price`) 
        ORDER BY `productId` ASC SEPARATOR '*') AS concat_products
FROM
  `users`
LEFT JOIN `products` 
ON `users`.`accountBalance` >= `product`.`price`
GROUP BY `productId` 

according to mysql manual

Values in VARCHAR columns are variable-length strings
Length can be specified as a value from 1 to 255 before MySQL 4.0.2 and 0 to 255 as of MySQL 4.0.2.

edit:

Values in VARCHAR columns are variable-length strings. The length can be specified as a value from 0 to 65,535.

1. Why mysql specifies more than 255 characters for varchar concat_products column?(solved!)

2. Why uf8_bin instead of utf8_general_ci?

3. Is it possible to change the data type of a column in a view for example in my case to text for concat_products column?

4. If not what can i do to prevent mysql from truncating concat_products column?

回答1:

As I already wrote in an earlier comment, the MySQL manual says:

Values in VARCHAR columns are variable-length strings. The length can be specified as a value from 0 to 65,535.

So the problem is not with the data type of the field.

The MySQL manual also says:

The result is truncated to the maximum length that is given by the group_concat_max_len system variable, which has a default value of 1024. The value can be set higher, although the effective maximum length of the return value is constrained by the value of max_allowed_packet. The syntax to change the value of group_concat_max_len at runtime is as follows, where val is an unsigned integer: SET [GLOBAL | SESSION] group_concat_max_len = val;

Your options for changing the value of group_concat_max_len are:

1. changing the value at MySQL startup by appending this to the command:
   --group_concat_max_len=your_value_here
2. adding this line in your MySQL configuration file (mysql.ini): group_concat_max_len=your_value_here
3. running this command after MySQL startup:
   SET GLOBAL group_concat_max_len=your_value_here;
4. running this command after opening a MySQL connection:
   SET SESSION group_concat_max_len=your_value_here;

Documentation: SET, Server System Variables: group_concat_max_len

回答2:

As Jocelyn mentioned, the size of a GROUP_CONCAT() result is bounded by group_concat_max_len, however there is an additional interaction with ORDER BY that results in a further truncation to 1/3 of group_concat_max_len. For an example, see this related answer.

The default value for group_concat_max_len is 1024, and 1024 / 3 = 341 probably explains why the type of concat_products shows up as varchar(341) in the original example. If you were to remove the GROUP BY productId clause, concat_products should show up as varchar(1024).

I have not found this interaction between GROUP_CONCAT() and ORDER BY mentioned in the MySQL Manual, but it affects at least MySQL Server 5.1.