UPDATE using JOIN appends only first row value to another table, why? - php

Question

I have a target table which crossed over 1M rows. Each time I will be getting 50K rows which may contain multiple duplicated entries. Hence I have decided to store CSV data into a temp table, then from temp_table to target_table by comparing rows between two tables...
If duplicated entries found append data from temp_table to target_table else Insert into the table... I am using partition here, so ON DUPLICATE key update is not working here.. in temp_table I am not using any KEYS
I have two tables which look like below
temp_table
Name | Type
John | Civil
John | Mech
target_table
Name | Type
John | Civil
When I run below query, I am getting an output of single row
UPDATE target_table JOIN temp_table
ON temp_table.Name = target_table.Name
SET target_table.Type = IF((LOCATE(temp_table.Type, target_table.Type) > 0)
target_table.Type,CONCAT(target_table.Type,',',temp_table.Type))
target_table
Name | Type
John | Civil
I am expecting output to be like below
target_table
Name | Type
John | Civil, Mech
May I know where it went wrong?

Answer 1

you should use a group_concat and use a subquery in join
UPDATE target_table
JOIN (
select name, group_concat(Type) grouped
from temp_table
group by name
) t ON t.Name = target_table.Name
SET target_table.Type = t.grouped

Answer 2

I suspect (but don't know for sure) and hopefully someone who does know will jump in and correct me, that an update join does not create a cartesian product in the way that a select would. As an attempted proof
truncate table temp_table;
insert into temp_table values
( 'John' , 'mech' ),
( 'John' , 'abc' );
truncate table target_table;
insert into target_table values
('john', 'civil', 9 );
UPDATE target_table JOIN temp_table
ON temp_table.Name = target_table.Name
set target_table.type = (concat(target_table.Type,',',temp_table.Type));
select * from target_table;
+------+------------+------+
| Name | Type | LOC |
+------+------------+------+
| john | civil,mech | 9 |
+------+------------+------+
1 row in set (0.00 sec)
note that abc from temp_table is ignored and mech is selected purely by chance.
if we change the order in temp_table
truncate table temp_table;
insert into temp_table values
( 'John' , 'abc' ),
( 'John' , 'mech' );
truncate table target_table;
insert into target_table values
('john', 'civil', 9 );
UPDATE target_table JOIN temp_table
ON temp_table.Name = target_table.Name
set target_table.type = (concat(target_table.Type,',',temp_table.Type));
select * from target_table;
we get
+------+-----------+------+
| Name | Type | LOC |
+------+-----------+------+
| john | civil,abc | 9 |
+------+-----------+------+
1 row in set (0.02 sec)
where abc is picked purely by chance.
In my view the safest way to do this is on a row by row basis ie a cursor.