mysql group_concat

group_concat，返回group中非null值连接起来的字符串。

• 默认分割符是逗号，使用separator指定分隔符。
• 用distinct过滤重复值。
• 用order by排序，默认是升序，asc明确升序；desc降序

GROUP_CONCAT(expr)

This function returns a string result with the concatenated non-NULL
values from a group. It returns NULL if there are no non-NULL values.
The full syntax is as follows:

GROUP_CONCAT([DISTINCT] expr [,expr ...]
             [ORDER BY {unsigned_integer | col_name | expr}
                 [ASC | DESC] [,col_name ...]]
             [SEPARATOR str_val]) 

        SELECT student_name,
         GROUP_CONCAT(test_score)
        FROM student
         GROUP BY student_name;

或者

       SELECT student_name,
       GROUP_CONCAT(DISTINCT test_score
                   ORDER BY test_score DESC SEPARATOR ' ')
         FROM student
         GROUP BY student_name; 

In MySQL, you can get the concatenated values of expression combinations. To eliminate duplicate values, use the DISTINCT clause. To sort values in the result, use the ORDER BY clause. To sort in reverse order, add the DESC (descending) keyword to the name of the column you are sorting by in the ORDER BY clause. The default is ascending order; this may be specified explicitly using the ASC keyword. The default separator between values in a group is comma (“,”). To specify a separator explicitly, use SEPARATOR followed by the string literal value that should be inserted between group values. To eliminate the separator altogether, specify SEPARATOR ”. 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; 

The return value is a nonbinary or binary string, depending on whether the arguments are nonbinary or binary strings. The result type is TEXT or BLOB unless group_concat_max_len is less than or equal to 512, in which case the result type is VARCHAR or VARBINARY.