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MySQL | Group_CONCAT() Function

  • Difficulty Level : Expert
  • Last Updated : 16 Aug, 2021

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.

Syntax:

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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:

  • Distinct: It eliminates the repetition 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’.

Examples:



Let, consider an “Employee” table:

emp_idfnamelnamedept_idstrength
1mukeshgupta2Leadership
3neelamsharma3Hard-working
1mukeshgupta2Responsible
2deveshtyagi2Punctuality
3neelamsharma3Self-motivated
1mukeshgupta2Quick-learner
4keshavsinghal3Listening
2deveshtyagi2Quick-learner
5tanyajain1Hard-working
4keshavsinghal3Critical thinking
5tanyajain1Goal-oriented

Queries:

1. Using simple GROUP_CONCAT() function-

SELECT emp_id, fname, lname, dept_id, 
GROUP_CONCAT ( strength ) as "strengths" 
FROM employee group by emp_id;

Output:

emp_idfnamelnamedept_idstrengths
1mukeshgupta2Leadership, Responsible, Quick-learner
2deveshtyagi2Punctuality, Quick-learner
3neelamsharma3Hard-working, Self-motivated
4keshavsinghal3Listening, Critical thinking
5tanyajain1Hard-working, Goal-oriented

2. Using a DISTINCT clause-

SELECT dept_id, 
GROUP_CONCAT ( DISTINCT strength) 
as "employees strengths"  
from employee group by dept_id;

Output:

dept_idemployees strengths
1Goal-oriented, Hard-working
2Leadership, Punctuality, Quick-learner, Responsible
3Critical thinking, Hard-working, Listening, Self-motivated

3. 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.



Output:

dept_idemployees ids
15
21, 2
33, 4

How to concatenate multiple rows of different columns in a single field.
Till 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.

Example:
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;

Explanation:
The above query consists 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-

dept_idstrengths
21: Leadership, Responsible, Quick-learner
22: Punctuality, Quick-learner
33: Hard-working, Self-motivated
34: Listening, Critical thinking
15: Hard-working, Goal-oriented

The outer SELECT statement will now group these rows according to “dept_id”.

Output:

dept_idemp-id : strengths
15: Hard-working, Goal-oriented
21: Leadership, Responsible, Quick-learner 2:Punctuality, Quick-learner
33: 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 system 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.
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