SQL merge row values in a column

Question

TABLE 1

CODE	VALUE
AA	21
AA	32
AB	24
AC	22
AA	42
AC	20
AD	27
AD	28
AE	29
AF	31
AG	45
AE	51
AH	33

TABLE 2

CODE	KEY
AA	1
AB	2
AC	3
AD	4
AE	5
AF	6
AG	7
AH	8

Here i want to transform the table in such a way that AB value is summed to AA without effecting its key similarly AG's value has to be summed with AD without changing its key as below. Table 1 can have multiple values for a single code. pls help or tell me the approach.

CODE	VALUE	KEY
AA	119	1
AC	22	3
AD	100	4
AE	29	5
AF	31	6
AH	33	8

Answer 1

Use a CASE expression in TABLE1 to merge the codes that you want and aggregate and join to TABLE2:

SELECT t1.code, t1.value, t2.key
FROM (
  SELECT CASE code 
           WHEN 'AB' THEN 'AA' 
           WHEN 'AG' THEN 'AD' 
           ELSE code 
         END code, 
         SUM(value) "value"
  FROM table1
  GROUP BY 1
) t1 INNER JOIN table2 t2
ON t2.code = t1.code
ORDER BY t1.code;

See the demo.