The GROUP_CONCAT() function in MySQL is used to concatenate data from multiple rows into one field. This is an aggregate (GROUP BY) function which returns a String value, if the group contains at least one non-NULL value. Otherwise, it returns NULL.
SELECT col1, col2, ..., colN GROUP_CONCAT ( [DISTINCT] col_name1 [ORDER BY clause] [SEPARATOR str_val] ) FROM table_name GROUP BY col_name2; col1, col2, ...colN : These are the column names of table. col_name1: Column of the table whose values are concatenated into a single field for each group. table_name: Name of table. col_name2: Column of the table according to which grouping is done.
Use of various clauses inside GROUP_CONCAT() function:
- Dinstinct: It eliminates repeatition of values from result.
- Order By: It sort the values of group in specific order and then concatenate them.
- Separator: By default, the values of group are separated by (, ) operator. In order to change this separator value, Separator clause is used followed by a string literal. It is given as Separator ‘str_value’.
Let, consider an “Employee” table:
- Using simple GROUP_CONCAT() function-
SELECT emp_id, fname, lname, dept_id, GROUP_CONCAT ( strength ) as "strengths" FROM employee group by emp_id;
emp_id fname lname dept_id strengths 1 mukesh gupta 2 Leadership, Resposible, Quick-learner 2 devesh tyagi 2 Punctuality, Quick-learner 3 neelam sharma 3 Hard-working, Self-motivated 4 keshav singhal 3 Listening, Critical thinking 5 tanya jain 1 Hard-working, Goal-oriented
- Using a DISTINCT clause-
SELECT dept_id, GROUP_CONCAT ( DISTINCT strength) as "employees strengths" from employee group by dept_id;
dept_id employees strengths 1 Goal-oriented, Hard-working 2 Leadership, Punctuality, Quick-learner, Responsible 3 Critical thinking, Hard-working, Listening, Self-motivated
- Use of ORDER BY clause:
SELECT dept_id, GROUP_CONCAT ( DISTINCT emp_id ORDER BY emp_id SEPARATOR', ') as "employees ids" from employee group by dept_id;
Here, Separator ', ' will separate the values by a comma (, ) and a whitespace character.
dept_id employees ids 1 5 2 1, 2 3 3, 4
How to concatenate multiple rows of different columns in a single field.
Til now we have seen the use of GROUP_CONCAT() function to group the values of multiple rows that belongs to same column. But, using concat() function and group_concat() function together, we can combine more than one column values of different rows into single field.
Considering above table “employee”, if we wish to find employees strength along with employees id in second query then it is written as-
SELECT dept_id, GROUP_CONCAT ( strengths SEPARATOR ' ') as "emp-id : strengths" FROM ( SELECT dept_id, CONCAT ( emp_id, ':', GROUP_CONCATt(strength SEPARATOR', ') ) as "strengths" FROM employee GROUP BYy emp_id )as emp GROUP BY dept_id;
The above query consist of two SELECT statements an inner one and the outer one.
The inner SELECT statement –
SELECT dept_id, concat ( emp_id, ':', GROUP_CONCAT ( strength separator ', ' ) ) as "strengths" FROM employee GROUP BY emp_id
It will group the rows of employee table according to “emp_id”. The first resulted column displays dept_id, second column displays emp_id along with their strengths list.
Output for inner SELECT statement-
|2||1: Leadership, Responsible, Quick-learner|
|2||2: Punctuality, Quick-learner|
|3||3: Hard-working, Self-motivated|
|3||4: Listening, Critical thinking|
|1||5: Hard-working, Goal-oriented|
The outer SELECT statement will now group these rows according to “dept_id”.
|dept_id||emp-id : strengths|
|1||5: Hard-working, Goal-oriented|
|2||1: Leadership, Responsible, Quick-learner 2:Punctuality, Quick-learner|
|3||3: Hard-working, Self-motivated 4:Listening, Critical thinking|
Note: The result of GROUP_CONCAT() function is truncated to the maximum length i.e 1024 which is given by sytem variable group_concat_max_len. However, the value of group_concat_max_len variable can be changed at runtime by using SETcommand as-
SET [GLOBAL | SESSION] group_concat_max_len = value; value: It is the new value set to the variable.