如何获取多条记录的第一个值？ （MySQL的）

Question

有两个表，一个是“用户”，另一个是“放映” 用户可以在筛选中具有多个记录。 我需要从一个多重筛选记录中获取最早记录的日期。 并且看看自那以后最早的一次放映记录是否已经过了30天。 我该怎么做？

我得到了这样的查询：

   SELECT users.id AS user_id, users.first_name, users.last_name, clients.name AS client, 
    users.social_security_number AS ssn, users.hiredate,  
    screening.id AS screening_id, screening.date AS screening_date, screening.maileddate AS screening_maileddate
    FROM users  
    INNER JOIN clients
    ON(
       users.client_id = clients.id
       AND users.client_id = '1879'
       )
    LEFT JOIN screening
    ON(
       users.id = screening.user_id
    )
    WHERE 
    (users.hiredate BETWEEN '2011-01-01' AND '2011-08-14'
    OR users.hiredate IS NULL)
    AND users.id IN(        
    SELECT users.id from users
    INNER JOIN screening sall
    ON(
      sall.user_id = users.id 
    )
    INNER JOIN screening s1
    ON(
      s1.user_id = users.id

      AND s1.date BETWEEN '2011-05-15' AND '2011-11-15'
    ) 
    INNER JOIN screening s2
    ON(
      s2.user_id = users.id

      AND s2.date BETWEEN '2011-05-15' AND '2011-11-15'
    ) 
    INNER JOIN screening s3
    ON(
      s3.user_id = users.id

      AND s3.date BETWEEN '2011-05-15' AND '2011-11-15'
    )
    INNER JOIN screening s4 
    ON(
      s4.user_id = users.id

      AND s4.date BETWEEN '2011-05-15' AND '2011-11-15'
    )
    INNER JOIN screening s5
    ON(
      s5.user_id = users.id

      AND s5.date BETWEEN '2011-05-15' AND '2011-11-15'
   )
    INNER JOIN screening s6
    ON(
      s6.user_id = users.id

      AND s6.date BETWEEN '2011-05-15' AND '2011-11-15'
   )
   WHERE (s1.cholesterol IS NOT NULL
   AND s2.ldl IS NOT NULL
   AND s3.triglycerides IS NOT NULL)
   AND (s4.glucose IS NOT NULL 
   OR s5.ha1c IS NOT NULL)
  )

    GROUP BY users.id

Answer 1

使用FIRST（column_name）命令获取第一条记录的值。

http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html