MySQL merging rows with equivalent ID - mysql

Question

I have an issue where a mistake resulted in a database table having both emails and GUIDs mixed in the ID column.
The table looks like this (simplified):
GUID | Value | Date
cf#a | 21 | 2016
mf#b | 42 | 2015
mf#b | 21 | 2016
1aXd | 3 | 2016
a7vf | 9 | 2015
Where for example user cf#a and user 1aXd are the same. The GUID - Email combinations are stored in another table. The emails are unique. Primary key is the ID(GUID) and the Date combined. My problem is, how do i update the table and merge the rows? the Value column of two merging rows should be summed.
So the example table assuming (cf#a -> 1aXd and mf#b -> 8bga and ui#q -> a7vf) would become:
GUID | Value | Date
1aXd | 24 | 2016 <-- merged, Value = 21+3
8bga | 42 | 2015 <-- converted to GUID
8bga | 21 | 2016 <-- converted to GUID
<-- one row removed (merged with the first one)
a7vf | 9 | 2015 <-- untouched
Thank you for any help!
I could do this in C# but i would rather learn how to do it with the MySQL Workbench

Answer 1

Use JOIN:
SELECT t1.Value + t2.Value
FROM t1
JOIN t2 USING (`GUID`)
If you want update values, you need something like this:
UPDATE t1
JOIN t2 USING (`GUID`)
SET t1.Value = t1.Value + t2.Value
Removing merged rows:
DELETE t2 FROM t2
JOIN t1 USING (`GUID`)
UPDATE
If has only one table.
Merge:
UPDATE t1
JOIN (
SELECT GUID, SUM(Value) as amount, COUNT(1) as cnt
FROM t1
GROUP BY `GUID`
HAVING cnt > 1
) t2 ON t2.GUID = t1.GUID
SET t1.Value = t2.amount;
Delete:
CREATE table t2 (
GUID integer,
Value integer,
Date integer
);
INSERT INTO t2 (GUID, Value, Date)
SELECT GUID, Value, MAX(Date) FROM t1 GUID, Value;
Result will be in t2.