Question

Is there a more efficient way of doing this? I do not want to update more than one record and it is possible that the two initial queries can match two different records.

SET @AppID = (SELECT appID FROM employees WHERE ssn = vSSN);

IF @AppID IS NULL THEN
    SET @AppID = (SELECT appID from applications WHERE appDate = vDate);
END IF;

UPDATE applications
SET status = vStatus
WHERE appID = @AppID;

Answer 1

You can use the COALESCE operator and the UPDATE/REPLACE statement. To be more precise, COALESCE picks the first non-null value out of a comma-separated list.

http://dev.mysql.com/doc/refman/5.5/en/comparison-operators.html#function_coalesce

http://dev.mysql.com/doc/refman/5.5/en/update.html

http://dev.mysql.com/doc/refman/5.5/en/replace.html

First, the UPDATE alternative:

UPDATE applications AS app
INNER JOIN (
SELECT COALESCE(employees.appID, applications.appID) AS appID
FROM applications
LEFT OUTER JOIN employees
ON employees.ssn = vSSN
WHERE applications.appDate = vDate
-- do some ordering or filtering if the date is not unique 
LIMIT 1) AS app2
ON app.appID = app2.appID
SET app.status = vStatus

Second, the REPLACE alternative. Please note that REPLACE first deletes the old entry and then reinserts the new one. Therefore, employees.appID may not be a foreign key of applications.appID.

REPLACE INTO applications (appID, status)
SELECT COALESCE(employees.appID, app2.appID) AS appIdToModify, vStatus
FROM applications AS app2
LEFT OUTER JOIN employees
ON ssn = vSSN
WHERE app2.appDate = vDate