Francesco Tisiot

Posted on Feb 13 • Originally published at ftisiot.net

How to CONCAT in MySQL

#mysql #concatenation #concat #string

One of the most common tasks with strings is concatenation! This blog post showcases several techniques to perform it with MySQL.

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String concatenation in MySQL by placing strings next to each other

Let's take a basic example: a table containing firstname and surname fields we want to concatenate:

CREATE TABLE CLIENTS (
    id INT PRIMARY KEY, 
    firstname VARCHAR(100), 
    surname VARCHAR(100),
    region VARCHAR(100),
    creation_date date,
    preferred_item VARCHAR(100)
    );
INSERT INTO CLIENTS VALUES 
    (1, 'John', 'Doe', 'EMEA', '2024-01-01', 'football'),
    (2, 'Laura', 'Smith', 'EMEA', '2024-02-01', 'tennis ball'),
    (3, 'Eva', 'Book', 'APAC', '2024-01-03', null),
    (4, 'Andy', 'Novel', 'APAC', '2024-02-05', 'teddy bear'),
    (5, 'Eva', 'Leeds', 'APAC', '2024-02-01', 'football');

We can concatenate two quoted strings by placing one string next to the other like:

select 'MyFirstname' 'MySurname';

The result is

+----------------------+
| MyFirstname          |
+----------------------+
| MyFirstnameMySurname |
+----------------------+
1 row in set (0.03 sec)

We can see that:

the result is the correct concatenation
the column name is the first quoted string

However, if we apply the same principle to standard columns instead of quoted strings with:

select firstname surname from CLIENTS;

We get a different result:

+---------+
| surname |
+---------+
| John    |
| Laura   |
| Eva     |
| Andy    |
| Eva     |
+---------+
5 rows in set (0.02 sec)

The result showcases only the firstname in the results, with the surname being included as the result column heading.

TLDR: concatenating strings in MySQL by placing strings next to each other is possible only if we are dealing with quoted strings.

Concatenating a non string

Is it possible with the same method to concatenate a non string?

select 'MyFirstname' 1;

Nope, we get ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '1' at line 1

Concatenating a NULL

What about concatenating a NULL value?

select 'MyFirstname' NULL;

Same error ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'NULL' at line 1

String concatenation in MySQL with the `||` operator

We can also concatenate firstname and surname using the || operator with:

select firstname || surname from CLIENTS;

The result will be:

+-----------------------+
| firstname || surname  |
+-----------------------+
| JohnDoe               |
| LauraSmith            |
| EvaBook               |
| AndyNovel             |
| EvaLeeds              |
+-----------------------+
5 rows in set (0.02 sec)

Please check that there's no space between the firstname and surname, but we can add it with:

select firstname || ' ' || surname from CLIENTS;

Which shows the expected result

+------------------------------+
| firstname || ' ' || surname  |
+------------------------------+
| John Doe                     |
| Laura Smith                  |
| Eva Book                     |
| Andy Novel                   |
| Eva Leeds                    |
+------------------------------+
5 rows in set (0.02 sec)

Concatenating a non string

What about concatenating strings with other fields that are not a string? If, for example, we try to concat the firstname and id (an integer) with the following:

select firstname || id from CLIENTS;

We get the expected result

+------------------+
| firstname || id  |
+------------------+
| John1            |
| Laura2           |
| Eva3             |
| Andy4            |
| Eva5             |
+------------------+

The same if we try to concatenate the creation_date date field

select firstname || creation_date from CLIENTS;

Result:

+-----------------------------+
| firstname || creation_date  |
+-----------------------------+
| John2024-01-01              |
| Laura2024-02-01             |
| Eva2024-01-03               |
| Andy2024-02-05              |
| Eva2024-02-01               |
+-----------------------------+
5 rows in set (0.02 sec)

However we might want to add a function to the date to get alternative formats from the default, for example the DATE_FORMAT function below converts the date in a weird format separating years, months and days with ..

select firstname || DATE_FORMAT(creation_date, '..%Y..%M..%D') from CLIENTS;

Results

+---------------------------------------------------------+
| firstname || DATE_FORMAT(creation_date, '..%Y..%M..%D') |
+---------------------------------------------------------+
| John..2024..January..1st                                |
| Laura..2024..February..1st                              |
| Eva..2024..January..3rd                                 |
| Andy..2024..February..5th                               |
| Eva..2024..February..1st                                |
+---------------------------------------------------------+
5 rows in set (0.03 sec)

Concatenating a NULL value

What does it happen if we try to concatenate a NULL value? If you check the dataset created, the row with id=3 for Eva Book has a preferred_item equal to NULL, let's check the output of concatenating the firstname, surname and preferred_item:

select firstname || surname || preferred_item  from CLIENTS;

Result

+------------------------------------------+
| firstname || surname || preferred_item   |
+------------------------------------------+
| JohnDoefootball                          |
| LauraSmithtennis ball                    |
| NULL                                     |
| AndyNovelteddy bear                      |
| EvaLeedsfootball                         |
+------------------------------------------+
5 rows in set (0.03 sec)

The result shows that the entire concatenation string is NULL!

TLDR: a concatenation will return NULL if any of the strings is NULL

String concatenation in MySQL with the `CONCAT` function

Another option is to use MySQL Concat function. Let's try with the basic firstname and surname example:

select concat(firstname, surname) from CLIENTS;

The results are in line with the previous || example, not having any additional separator added into the resulting string.

+----------------------------+
| concat(firstname, surname) |
+----------------------------+
| JohnDoe                    |
| LauraSmith                 |
| EvaBook                    |
| AndyNovel                  |
| EvaLeeds                   |
+----------------------------+
5 rows in set (0.03 sec)

How can we add a space in between? We have two options:

Add a space to the CONCAT function

select concat(firstname, ' ' ,surname) from CLIENTS;

Use the CONCAT_WS function (concat with separator) passing the space as separator

select concat_ws(' ', firstname, surname) from CLIENTS;

Concatenating a non string with the CONCAT function

What happens if we include non strings in the CONCAT function?

select concat(firstname, id) from CLIENTS;

As before we get the expected result

+-----------------------+
| concat(firstname, id) |
+-----------------------+
| John1                 |
| Laura2                |
| Eva3                  |
| Andy4                 |
| Eva5                  |
+-----------------------+
5 rows in set (0.03 sec)

Concatenating a NULL value with the CONCAT function

What about NULL values?

select concat(firstname, surname, preferred_item)  from CLIENTS;

As before, if any of the source strings is NULL, the result of the concatenation is NULL

+--------------------------------------------+
| concat(firstname, surname, preferred_item) |
+--------------------------------------------+
| JohnDoefootball                            |
| LauraSmithtennis ball                      |
| NULL                                       |
| AndyNovelteddy bear                        |
| EvaLeedsfootball                           |
+--------------------------------------------+
5 rows in set (0.03 sec)

String aggregation via concatenation in MySQL with the `GROUP_CONCAT` function

If we need to concatenate all the strings belonging to the same group, we can use MySQL GROUP_CONCAT function.

If, in our example, we want to concatenate the firstname of all the people grouped by their region we can write the following:

select region,
    group_concat(firstname)
from
    CLIENTS
group by region

The result is the following, with , as the default separator:

+--------+-------------------------+
| region | group_concat(firstname) |
+--------+-------------------------+
| APAC   | Eva,Andy,Eva            |
| EMEA   | John,Laura              |
+--------+-------------------------+
2 rows in set (0.03 sec)

We can also order how the strings are ordered in the list with:

select region,
    group_concat(firstname ORDER BY firstname)
from
    CLIENTS
group by region;

Result

+--------+--------------------------------------------+
| region | group_concat(firstname ORDER BY firstname) |
+--------+--------------------------------------------+
| APAC   | Andy,Eva,Eva                               |
| EMEA   | John,Laura                                 |
+--------+--------------------------------------------+
2 rows in set (0.03 sec)

And change the separator with

select region,
    group_concat(firstname ORDER BY firstname SEPARATOR '$$') res
from
    CLIENTS
group by region;

Result

+--------+----------------+
| region | res            |
+--------+----------------+
| APAC   | Andy$$Eva$$Eva |
| EMEA   | John$$Laura    |
+--------+----------------+
2 rows in set (0.02 sec)

We can also only list the distinct values, removing the duplication of Eva with:

select region,
    group_concat(DISTINCT firstname ORDER BY firstname SEPARATOR '$$') res
from
    CLIENTS
group by region;

Result

+--------+-------------+
| region | res         |
+--------+-------------+
| APAC   | Andy$$Eva   |
| EMEA   | John$$Laura |
+--------+-------------+
2 rows in set (0.03 sec)

Aggregating and concatenating a non string with the GROUP_CONCAT function

Let's try with a non string, like the id field:

select region,
    group_concat(id) res
from
    CLIENTS
group by region;

Works!

+--------+-------+
| region | res   |
+--------+-------+
| APAC   | 3,4,5 |
| EMEA   | 1,2   |
+--------+-------+
2 rows in set (0.02 sec)

The same if we try with creation_date

+--------+----------------------------------+
| region | res                              |
+--------+----------------------------------+
| APAC   | 2024-01-03,2024-02-05,2024-02-01 |
| EMEA   | 2024-01-01,2024-02-01            |
+--------+----------------------------------+
2 rows in set (0.02 sec)

Aggregating and concatenating a NULL value with the GROUP_CONCAT function

What about our NULL value? Let's check it out!

select region,
    group_concat(preferred_item) res
from
    CLIENTS
group by region;

In this case the behaviour is different:

+--------+----------------------+
| region | res                  |
+--------+----------------------+
| APAC   | teddy bear,football  |
| EMEA   | football,tennis ball |
+--------+----------------------+
2 rows in set (0.02 sec)

Even if the row with id=3 part of the APAC group had preferred_item equal to NULL, the resulting aggregate is not NULL! This is a common behaviour for aggregate functions to ignore the NULL values.

Conclusion

Here's an handy overview of the concatenation possibilities in MySQL:

Method	code	works with quoted strings	works with standard columns	works with non strings	Works with `NULL`	aggregates results
Placing strings next to each other	'a' 'b'	✅	❌	❌	❌	❌
\| operator	'a' \|\| 'b'	✅	✅	✅	✅	❌
CONCAT function	concat('a','b')	✅	✅	✅	✅	❌
GROUP_CONCAT function	group_concat('a','b')	✅	✅	✅	✅	✅

DEV Community

How to CONCAT in MySQL

String concatenation in MySQL by placing strings next to each other

Concatenating a non string

Concatenating a NULL

String concatenation in MySQL with the `||` operator

Concatenating a non string

Concatenating a NULL value

String concatenation in MySQL with the `CONCAT` function

Concatenating a non string with the CONCAT function

Concatenating a NULL value with the CONCAT function

String aggregation via concatenation in MySQL with the `GROUP_CONCAT` function

Aggregating and concatenating a non string with the GROUP_CONCAT function

Aggregating and concatenating a NULL value with the GROUP_CONCAT function

Conclusion

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String concatenation in MySQL by placing strings next to each other

Concatenating a non string

Concatenating a NULL

String concatenation in MySQL with the || operator

Concatenating a non string

Concatenating a NULL value

String concatenation in MySQL with the CONCAT function

Concatenating a non string with the CONCAT function

Concatenating a NULL value with the CONCAT function

String aggregation via concatenation in MySQL with the GROUP_CONCAT function

Aggregating and concatenating a non string with the GROUP_CONCAT function

Aggregating and concatenating a NULL value with the GROUP_CONCAT function

Conclusion

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