使用1个查询使用MySQL连接3个表

Question

This has been asked and answered in different instances but all that I've seen doesn't quite work for my problem.

PROBLEM: I have 3 tables I need to pull data from at the same time to compare and retrieve information from. All 3 tables contain an 'email' column. Now, a user's email from table1 may match same user's email in both table2 AND table3, depending on user's status. OR an email from table1 will ONLY match either an email in table2 or table3, depending on user's status again. For example, a user may have a red status (user will show in table2), a blue status (user will show in table3), or both, red and blue (user will show in both, table2 and table3).

WHAT IS NEEDED: an email from table1 needs to be compared to email in table2 and table3 and return a region value for a given user, which is recorded in table2 and table3 but not in table1. I know. Delightful data architecture! Either way, I was able to JOIN table1 to table2 very successfully but I am not sure how to slap on a JOIN with table3.

Here's the query for 2 tables:

SELECT * FROM table1
INNER JOIN table2
ON table2.email = table1.email
WHERE month = 'numberHere' ORDER BY submitdate DESC

When I simply add another INNER JOIN my code doesn't break per say but it doesn't give me any rows to be displayed either. So the code below doesn't work despite the many working examples from the web:

SELECT * FROM table1
INNER JOIN table2
ON table2.email = table1.email
INNER JOIN table3
ON table3.email = table2.email
WHERE month = 'numberHere' ORDER BY submitdate DESC

Answer 1

You need to use LEFT JOIN , so that the joins will succeed even if there's no matching row in one of the tables.

SELECT * FROM table1
LEFT JOIN table2
    ON table2.email = table1.email
LEFT JOIN table3
    ON table3.email = table2.email
WHERE month = 'numberHere'
    AND (table2.email IS NOT NULL OR table3.email IS NOT NULL)
ORDER BY submitdate DESC

The additional conditions filter out rows that have no match in either of the tables.

Answer 2

To match the row from table1 in any case and the rows of other tables only if they are, you have to use LEFT JOIN , joining table2 and table3 with table1 (not table2 with table1 and table3 with tabel2 ):

SELECT * FROM table1
LEFT JOIN table2
ON table2.email = table1.email
LEFT JOIN table3
ON table3.email = table1.email

In this way you select values from table1 even without a match in other tables.

See more about SQL Joins: Different SQL Joins