I have two tables as below. The Id column value in both the tables mentioned below is auto-incremented.  

**Group**  

`+----+-----------+----------------+`  

 | `Id |` `GroupId |` `GroupName |`  

`+----+-----------+----------------+`  

 | `1 |` `10 |` `Grp1@abc.com |`  

 | `2 |` `20 |` `Grp2@abc.com |`  

 | `3 |` `30 |` `Grp3@xyz.com |`  

 | `4 |` `40 |` `Grp4@def.com |`  

`+----+-----------+----------------+` 

**Customer**  

`+---+-----------------+------------+----------+---------------+`  

 | `Id |` `GroupAliasName |` `Domain |` `GroupId |` `CustomerName |`  

`+---+-----------------+------------+----------+---------------+`  

 | `1 |` `Grp1 |` `abc.com |` `null |` `Cust1 |`  

 | `2 |` `Grp2 |` `abc.com |` `null |` `Cust2 |`  

 | `3 |` `Grp3 |` `xyz.com |` `null |` `Cust3 |`  

 | `4 |` `Grp4 |` `def.com |` `null |` `Cust4 |`  

`+---+-----------------+------------+----------+---------------+` 

Now from **Customer** table `'GroupAliasName'` and `'Domain'` when concatenated as `'GroupAliasName@Domain'` is equivalent to `'GroupName'` in **Group** table.

Using the concatenated value from **Customer** table, I need to pull the `'GroupId'` from the **Group** table and populate the same in **Customer** table's `'GroupId'` as below 

**Customer**  

`+----+----------------+------------+----------+---------------+`  

 | `Id |` `GroupAliasName |` `Domain |` `GroupId |` `CustomerName |`  

`+----+----------------+-----------+---------+-----------------+`  

 | `1 |` `Grp1 |` `abc.com |` `10 |` `Cust1 |`  

 | `2 |` `Grp2 |` `abc.com |` `20 |` `Cust2 |`  

 | `3 |` `Grp3 |` `xyz.com |` `30 |` `Cust3 |`  

 | `4 |` `Grp4 |` `def.com |` `40 |` `Cust4 |`  

`+----+----------------+------------+----------+---------------+`

The query which I tried is as below 

`UPDATE Customer SET GroupId =
(SELECT GroupId FROM Group G
WHERE GroupName =
 (SELECT CONCAT(GroupAliasName, '@', Domain) AS GroupName
 FROM Customer
 WHERE Domain IS NOT NULL) AND G.GroupName = GroupName);`

But I get error as `'Subquery returns more than 1 row'`.  

Please suggest or provide your inputs.