PostgreSQL Selecting Most Recent Entry for a Given

问题:

Table Essentially looks like:

Serial-ID, ID, Date, Data, Data, Data, etc.

There can be Multiple Rows for the Same ID. I'd like to create a view of this table to be used in Reports that only shows the most recent entry for each ID. It should show all of the columns.

Can someone help me with the SQL select? thanks.

回答1:

There's about 5 different ways to do this, but here's one:

SELECT *
FROM yourTable AS T1 
WHERE NOT EXISTS(
    SELECT *
    FROM yourTable AS T2
    WHERE T2.ID = T1.ID AND T2.Date > T1.Date
)

And here's another:

SELECT T1.*
FROM yourTable AS T1
LEFT JOIN yourTable AS T2 ON
(
    T2.ID = T1.ID 
    AND T2.Date > T1.Date
)
WHERE T2.ID IS NULL

One more:

WITH T AS (
    SELECT *, ROW_NUMBER() OVER(PARTITION BY ID ORDER BY Date DESC) AS rn
    FROM yourTable
)
SELECT * FROM T WHERE rn = 1

Ok, i'm getting carried away, here's the last one I'll post(for now):

WITH T AS (
    SELECT ID, MAX(Date) AS latest_date
    FROM yourTable
    GROUP BY ID
)
SELECT yourTable.*
FROM yourTable
JOIN T ON T.ID = yourTable.ID AND T.latest_date = yourTable.Date

回答2:

This seems like a good use for correlated subqueries:

CREATE VIEW your_view AS
SELECT *
FROM your_table a
WHERE date = (
    SELECT MAX(date)
    FROM your_table b
    WHERE b.id = a.id
)

Your date column would need to uniquely identify each row (like a TIMESTAMP type).

回答3:

I would use DISTINCT ON

CREATE VIEW your_view AS
SELECT DISTINCT ON (id) *
FROM your_table a
ORDER BY id, date DESC;

This works because distinct on suppresses rows with duplicates of the expression in parentheses. DESC in order by means the one that normally sorts last will be first, and therefor be the one that shows in the result.

https://www.postgresql.org/docs/10/static/sql-select.html#SQL-DISTINCT