Question

我有一个3个MySQL表，我需要从中得到结果，以及：

1. Towns
Fields Towncode and Townname

2. Students
Fields student_id,name,surname,address,streetcode,towncode,HeadOfFamily

3. Phonebank
Fields student_id,contacted,towncode

现在我需要一个mysql语句（a）来获取学生表中的总户数，以及（b）联系该特定城镇的学生人数。

直到步骤（a）我管理了哪个并且非常快，这是：

SELECT 
      t.towncode as towncode, 
      t.townname as townname, 
      (SELECT COUNT(*) 
          FROM students p 
          WHERE p.towncode=t.towncode 
            and p.student_hh='H') AS households
   FROM 
      towns t 
   ORDER BY 
      t.towncode ASC

但我无法插入另一个SELECT STATEMENT来获取特定城镇的电话号码。

请你帮忙吗？

Answer 1

对城镇进行每记录计数可能会对您造成伤害。我会首先查询学生表，然后使用加入城镇的表。

对于索引，我会有以下索引

table      index
towns      ( towncode, townname )
students   ( student_hh, towncode )

SELECT 
      t.towncode as towncode, 
      t.townname as townname, 
      TownCnts.households
   FROM 
      towns t 
         JOIN ( SELECT 
                      p.towncode,
                      COUNT(*) households
                   from
                      students p
                   where
                      p.student_hh = 'H'
                   group by 
                      p.towncode ) as TownCnts
           ON t.towncode = TownCnts.towncode
   ORDER BY 
      t.towncode ASC

Answer 2

假设“call”的数量是具有特定城镇代码的PhoneBank表中的行数，那么您可以添加另一个子选择：

SELECT t.towncode as towncode, t.townname as townname, 
       (SELECT COUNT(*) 
        FROM students p 
        WHERE p.towncode = t.towncode and p.student_hh='H'
       ) AS households,
       (SELECT COUNT(*) 
        FROM phonebank pb
        WHERE pb.towncode = t.towncode 
       ) AS calls
FROM towns t 
ORDER BY t.towncode ASC ;

你的问题有点模糊。您可能希望在第二个查询中使用count(distinct studentid)，而不是count(*)。

要优化此查询，请创建以下索引：

镇（towncode）
学生（towncode，student_hh）
phonebank（towncode）

MySQL组运行速度太慢

2 个答案: