SQL filtering using an array

Question

I have the following tables in my database:

Table: Employee

ID      Name
--      ----
1       Mike
2       Peter
3       Daniel

Table: Location

EmployeeID      City
----------      ----
1               Berlin
1               Stuttgart
1               München
2               Hamburg
3               Stuttgart
3               Berlin

The Employee table contains information about the employees. The Location table contains information about the locations the employees have their projects in (e.g Mike has projects in Berlin, Stuttgart, and München).

What I want to do now is to filter employees by given locations in an array but still retrieve all the locations of each employee. For example, filtering by the array ["Berlin", Stuttgart] should return the following result:

ID      Name      City
--      ----      ----
1       Mike      Berlin, Stuttgart, München
3       Daniel    Stuttgart, Berlin

I know that I can use GROUP_CONCAT() to concatenate the locations. But how can I do the filtering? I need to be able to do it using SQL.

Answer 1

Apply WHERE condition to get EmployeeID from Location table:

SELECT 
  Employee.*, 
  GROUP_CONCAT(Location.City) 
FROM 
  Employee 
    INNER JOIN Location 
      ON Employee.ID=Location.EmployeeID 
    INNER JOIN 
      (SELECT DISTINCT 
        EmployeeID 
       FROM 
         Location 
       WHERE 
         City IN ('Berlin', 'Stuttgart')) AS IDS
      ON Employee.ID=IDS.EmployeeID
GROUP BY 
  Employee.ID

-check the fiddle.

Alternative solution would be to use HAVING clause with plain query and INSTR(), for example, but I don't recommend you to do it because comparing in HAVING will be slow and, besides, list of values will produce multiple comparisons.

Answer 2

You can use below query:

SELECT 
a.id,a.`name`,GROUP_CONCAT(b.city) 
FROM employee a 
JOIN 
(SELECT DISTINCT employeeid,city FROM location WHERE location IN ('Berlin', 'Stuttgart')) b
ON a.id=b.employeeid 
GROUP BY a.id;