Group_concat () function Summary

Group_concat (), Manual Description: This function returns a string with a non-NULL value from a group of connections. It is abstract and hard to understand. Generally, group_concat () calculates which rows belong to the same group and displays the columns belonging to the same group. The columns to be returned are determined by the function parameter (that is, the field name. The group must have a standard, that is, grouping based on the columns specified by group. The group_concat function should have executed the group by statement internally, which is my guess. 1. test statement: SELECT group_concat (town) FROM 'players' group by town to find out which values are the same in the town. If they are the same, all values are listed, use commas (,) to list data, as follows: group_concat (town) Beijing, Beijing Changsha 2. test: SELECT group_concat (town) FROM players result: group_concat (town) Changsha, Beijing, Beijing, can the above prove that group_concat can only be used together with the group by statement to produce results? The following is an actual test 3. influence of test Constants on group_concat () configuration: SET @ GROUP_CONCAT_MAX_LEN = 4 the syntax mentioned in the manual is SET [SESSION | GLOBAL] group_concat_max_len = val; what are the differences between the two types? SET @ global. GROUP_CONCAT_MAX_LEN = 4; global can be omitted. SET @ GROUP_CONCAT_MAX_LEN = 4; 4. Use the statement SELECT group_concat (town) FROM 'players '. The result is as follows: group_concat (town) Changsha, Beijing, Changsha, and Beijing. Conclusion: The group_concat () function must be used together with the group by statement to obtain the expected results. The reason can be understood as follows: group_concat () obtains all the members of Group x (the column parameters in the function specify the fields to be displayed ). Where does group x come from? If you do not specify group by, you do not know which group group_concat () displays the members. Therefore, when there is no group by clause above, Changsha and Beijing are displayed. When does this function actually need to be used? If the query result is as follows: the group name is displayed on the left and all members in the group are displayed on the right. With this function, you can save a lot of things. In addition, let me use: SELECT group_concat (name, sex) FROM 'players' town. Not significant. Group_concat () specifies a column, which is the best case. If multiple columns are specified. The result is similar to the following: group_concat (name, sex) Wang Yu, Wang Xiaoming, Liu huifemale, and Shu Ming female.