[mysql|mariadb] 문자열 합치는 함수 (feat. concat, concat_ws, group_concat)

concat 문자열 합치기

https://mariadb.com/kb/en/concat/

CONCAT

syntax

CONCAT(str1,str2,...)

example

SELECT CONCAT('Ma', 'ria', 'DB');
+---------------------------+
| CONCAT('Ma', 'ria', 'DB') |
+---------------------------+
| MariaDB                   |
+---------------------------+

SELECT CONCAT('Ma', 'ria', NULL, 'DB');
+---------------------------------+
| CONCAT('Ma', 'ria', NULL, 'DB') |
+---------------------------------+
| NULL                            |
+---------------------------------+

SELECT CONCAT(42.0);
+--------------+
| CONCAT(42.0) |
+--------------+
| 42.0         |
+--------------+

concat_ws 여러 컬럼값을 연결해서 하나의 컬럼값으로 출력하기 (feat. 중간 문자 삽입)

syntax

CONCAT_WS(separator,str1,str2,...)

example

SELECT CONCAT_WS(',','First name','Second name','Last Name');
+-------------------------------------------------------+
| CONCAT_WS(',','First name','Second name','Last Name') |
+-------------------------------------------------------+
| First name,Second name,Last Name                      |
+-------------------------------------------------------+

SELECT CONCAT_WS('-','Floor',NULL,'Room');
+------------------------------------+
| CONCAT_WS('-','Floor',NULL,'Room') |
+------------------------------------+
| Floor-Room                         |
+------------------------------------+

group_concat 여러줄을 한 컬럼값으로 출력하기

https://mariadb.com/kb/en/group_concat/

GROUP_CONCAT

syntax

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

LIMIT

MariaDB starting with 10.3.3

Until MariaDB 10.3.2, it was not possible to use the LIMIT clause with GROUP_CONCAT. This restriction was lifted in MariaDB 10.3.3.

example

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

Get a readable list of MariaDB users from the mysql.user table:

SELECT GROUP_CONCAT(DISTINCT User ORDER BY User SEPARATOR '\n')
   FROM mysql.user;

In the former example, DISTINCT is used because the same user may occur more than once. The new line (\n) used as a SEPARATOR makes the results easier to read.

Get a readable list of hosts from which each user can connect:

SELECT User, GROUP_CONCAT(Host ORDER BY Host SEPARATOR ', ') 
   FROM mysql.user GROUP BY User ORDER BY User;

The former example shows the difference between the GROUP_CONCAT's ORDER BY (which sorts the concatenated hosts), and the SELECT's ORDER BY (which sorts the rows).

From MariaDB 10.3.3, LIMIT can be used with GROUP_CONCAT, so, for example, given the following table:

CREATE TABLE d (dd DATE, cc INT);

INSERT INTO d VALUES ('2017-01-01',1);
INSERT INTO d VALUES ('2017-01-02',2);
INSERT INTO d VALUES ('2017-01-04',3);

the following query:

SELECT SUBSTRING_INDEX(GROUP_CONCAT(CONCAT_WS(":",dd,cc) ORDER BY cc DESC),",",1) FROM d;
+----------------------------------------------------------------------------+
| SUBSTRING_INDEX(GROUP_CONCAT(CONCAT_WS(":",dd,cc) ORDER BY cc DESC),",",1) |
+----------------------------------------------------------------------------+
| 2017-01-04:3                                                               |
+----------------------------------------------------------------------------+

can be more simply rewritten as:

SELECT GROUP_CONCAT(CONCAT_WS(":",dd,cc) ORDER BY cc DESC LIMIT 1) FROM d;
+-------------------------------------------------------------+
| GROUP_CONCAT(CONCAT_WS(":",dd,cc) ORDER BY cc DESC LIMIT 1) |
+-------------------------------------------------------------+
| 2017-01-04:3                                                |
+-------------------------------------------------------------+