MySQL将基于另一列的值在两个列中显示为“ 0”的两个表联接

Question

I have two tables."login" and "acc_reg_num".

"login" table has two columns: 'account_id' and 'userid'. (Using prefix "l")
"acc_reg_num" has three: 'accound_id', 'key' and 'value'. (Using prefix "a")

I need to join the column "a.value" to the "login" table. Depending on the value in "a.key": If "a.key" is "SAMPLEVALUE" then use the value in "a.value". But, if "a.key" is [Anything else] then place a "0" in "a.value" instead

I hope this is clear. My current Query looks like this but it's showing duplicate results and values in "a.value" regardless "a.key" is "SAMPLEVALUE" or not.

SELECT l.account_id, l.userid, a.value AS 'CashPoints'
FROM login AS l
LEFT JOIN acc_reg_num AS a ON a.account_id = l.account_id

I also need no duplicates for the l.account_id and l.userid columns.

Answer 1

You can add the a.key='SAMPLEVALUE' condition to the existing ON condition:

SELECT l.account_id, l.userid, COALESCE(a.value, 0) AS 'CashPoints'
FROM
  login AS l LEFT JOIN acc_reg_num AS a
  ON a.account_id = l.account_id AND a.key='SAMPLEVALUE'

in case there's no SAMPLEVALUE the join won't succeed and a.value will be NULL, then you can use COALESCE to get 0 instead of NULL.

Answer 2

You can use a CASE statement for this like

SELECT l.account_id, l.userid, 
CASE WHEN a.key = 'SAMPLEVALUE' THEN a.value ELSE '0' END AS 'CashPoints'
FROM login AS l
LEFT JOIN acc_reg_num AS a ON a.account_id = l.account_id