Home Database Mysql Tutorial Mysql函数GROUP_CONCAT

Mysql函数GROUP_CONCAT

Jun 07, 2016 pm 04:39 PM
co concat mysql function

GROUP_CONCAT()是MySQL数据库提供的一个函数,通常跟GROUP BY一起用,具体可参考MySQL官方文挡:http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_group-concat。语法: GROUP_CONCAT([DISTINCT]?expr?[,expr ...]?[ORDER?BY?{unsigne

GROUP_CONCAT()是MySQL数据库提供的一个函数,通常跟GROUP BY一起用,具体可参考MySQL官方文挡:http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_group-concat。 语法:
GROUP_CONCAT([DISTINCT]?expr?[,expr ...]?[ORDER?BY?{unsigned_integer | col_name | expr}?[ASC?|DESC]?[,col_name ...]]?[SEPARATOR str_val])
1.例如:
SELECT?student_id, GROUP_CONCAT(courses_id)?AS courses?FROM?student_courses?WHEREstudent_id=2?GROUP?BY?student_id;?
+------------+---------+?
| student_id | courses |?
+------------+---------+?
|?2?         |?3,4,5?|
+------------+---------+?
这 就不需要用php循环了
$row?=?$pdo->query("SELECT student_id, GROUP_CONCAT(courses_id) AS courses FROM student_courses WHERE student_id=2 GROUP BY student_id");
$result?=?explode(',',?$row['courses']);?
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2.当然分隔符还可以自定义,默认是以“,”作为分隔符,若要改为“|||”,则使用SEPARATOR来指定,例如:
SELECT?student_id, GROUP_CONCAT(courses_id SEPARATOR?'|||')?AS courses?FROM?student_coursesWHERE?student_id=2?GROUP?BY?student_id;
+------------+---------+?
| student_id | courses |?
+------------+---------+?
|?2         ?|?3|||4|||5?|
+------------+---------+?
3.除此之外,还可以对这个组的值来进行排序再连接成字符串,例如按courses_id降序来排:
SELECT?student_id, GROUP_CONCAT(courses_id?ORDER?BY?courses_id DESC)?AS courses?FROMstudent_courses?WHERE?student_id=2?GROUP?BY?student_id;
+------------+---------+?
| student_id | courses |?
+------------+---------+?
|?2?|?5,4,3?|
+------------+---------+?
4.需要注意的:
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a.int字段的连接陷阱 当你用group_concat的时候请注意,连接起来的字段如果是int型,一定要转换成char再拼起来, 否则在你执行后(ExecuteScalar或者其它任何执行SQL返回结果的方法)返回的将不是一个逗号隔开的串, 而是byte[]。 该问题当你在SQLyog等一些工具中是体现不出来的,所以很难发现。 select group_concat(ipaddress) from t_ip 返回逗号隔开的串 select group_concat(id) from t_ip 返回byte[] select group_concat(CAST(id as char)) from t_dep 返回逗号隔开的串 select group_concat(Convert(id , char)) from t_dep 返回逗号隔开的串 附Cast,convert的用法: CAST(expr AS type), CONVERT(expr,type) , CONVERT(expr USING transcoding_name) CAST() 和CONVERT() 函数可用来获取一个类型的值,并产生另一个类型的值。 这个类型 可以是以下值其中的 一个: BINARY[(N)] CHAR[(N)] DATE DATETIME DECIMAL SIGNED [INTEGER] TIME UNSIGNED [INTEGER] b.长度陷阱 用了group_concat后,select里如果使用了limit是不起作用的. 用group_concat连接字段的时候是有长度限制的,并不是有多少连多少。但你可以设置一下。 使用group_concat_max_len系统变量,你可以设置允许的最大长度。 程序中进行这项操作的语法如下,其中 val 是一个无符号整数: SET [SESSION | GLOBAL] group_concat_max_len = val; 若已经设置了最大长度, 则结果被截至这个最大长度。 在SQLyog中执行 SET GLOBAL group_concat_max_len = 10 后,重新打开SQLyog,设置就会生效。
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