如何选择两个字段唯一的记录

Question

I have a table that looks something like this:

+-------------+----------+---------------+------------+
| Contract_ID |   Name   |   Username    |  Password  |
+-------------+----------+---------------+------------+
|    12345671 | Facebook | john.doe      | password   |
|    12345672 | Google   | john.doe      | password   |
|    12345673 | Apple    | martha.malone | doodlebear |
|    12345674 | Samsung  | jimmy47       | parkour445 |
|    12345675 | HTC      | rick.grimes   | simpsons33 |
+-------------+----------+---------------+------------+

I'd like to select only records where there is a one to one ratio between username/password combination and contract id. In this case, that would mean I'd like to select the 3rd, 4th and 5th records in this table.

I've tried different combinations of the DISTINCT keyword but that doesn't seem to be the correct route to go. Is there a query that can return this information?

As a sort of bonus question: is there a query that can produce the opposite results (ie. only records where there is greater than 1 to 1 ratio between contract ids and username/password combination)?

Answer 1

You should use the GROUP BY clause together with the HAVING clause, for example:

SELECT Username, Password FROM Table
GROUP BY Username, Password
HAVING COUNT(*) = 1

The opposite is:

SELECT Username, Password FROM Table
GROUP BY Username, Password
HAVING COUNT(*) > 1

Answer 2

按（名称，密码）分组不起作用？

Answer 3

Very naughty way of doing it,not good at all, but it works:

    select distinct
    least(col1, col2) as value1
  , greatest(col1, col2) as value2
from yourtable