在SQL内部联接中使用别名

Question

I've had a look at a few examples of using joins to get the highest value in a group, but the methods I have tried don't like the use of aliases outside of the inner join.

SELECT f.year, f.name, f.date_start, f.date_end, f.max_year FROM ( SELECT EXTRACT(year FROM date_start) AS year, MAX(DATEDIFF(date_end,date_start)) AS max_year FROM mytable GROUP BY year ) AS x inner join mytable AS f on f.year = x.year and f.max_year = x.max_year;

So if I have a table:

Name date_start date_end John 1950-04-05 1960-07-08 Jack 1950-04-06 1960-12-31 Mark 1954-01-01 1970-01-01 Jane 1954-10-10 1978-10-01

Then I want it to bring up the entries that have the greatest gap between the two dates, for each year of the start_date:

Year Name date_start date_end max_year 1950 Jack 1950-04-06 1960-12-31 3922 1954 Jane 1954-10-10 1978-10-01 8758

Any suggestions of how to get around this?

Answer 1

It looks like you are using wrong aliases in your SELECT list, this should be better:

SELECT x.year, f.name, f.date_start, f.date_end, x.max_year FROM
(
    SELECT 
      EXTRACT(year FROM date_start) AS year, 
      MAX(DATEDIFF(date_end,date_start)) AS max_year
    FROM mytable GROUP BY year
)  AS x inner join
mytable AS f on  EXTRACT(year FROM f.date_start) = x.year 
AND DATEDIFF(f.date_end, f.date_start) = x.max_year;

However, I would do it like this:

SELECT name, date_start, date_end
FROM mytable f
WHERE NOT EXISTS (
  SELECT * FROM mytable 
  WHERE  
    EXTRACT(year FROM date_start) = EXTRACT(year FROM f.date_start) AND
    DATEDIFF(date_end, date_start) > DATEDIFF(f.date_end, f.date_start)
)

Answer 2

You can try with this:

SELECT * FROM
(
  SELECT *, EXTRACT(year FROM date_start) AS year, DATEDIFF(date_end,date_start) AS diff
  FROM mytable
  ORDER BY diff DESC
) sq
GROUP BY year
ORDER BY year ASC

Fiddle demo: http://sqlfiddle.com/#!2/4d0da/11