Left Join Where the right table has Having Count = 1

问题

Table 1:

ID     Values
1      1
2      2
3      3
4      4

Table 2:

ID    Values
1     1
1     2
1     2
2     4
4     5
4     6

I need a result to be:

ID    Table 1 Values    Table 2 Values
1     1                 1
2     2                 4
3     3                         *This is blank because Table 2 doesn't have ID 3
4     4                 5
4     4                 6

Notice the Exclusion of the rows in table 2 Where the Values are duplicated ( ID 1 and Value 2). Yet the 2 rows for ID 4 because both values from Table 2 for ID 4 do not have duplicity.

So this is a left join and I want only the Value from table 2 where that Value is not duplicated AND the ID matches an ID from Table 1.

回答1:

Use a CTE over table 2, and then use that as the right side of your left join:

WITH t2 AS (
    SELECT ID, Values FROM [Table 2]
    GROUP BY ID, Values
    HAVING COUNT(*) = 1
)
SELECT t1.ID, t1.Values [Table 1 Values], t2.Values [Table 2 Values]
    FROM [Table 1] t1
    LEFT JOIN t2 ON t1.ID = t2.ID

来源：https://stackoverflow.com/questions/15420520/left-join-where-the-right-table-has-having-count-1