MySQL Group運行太慢

Question

我有3個MySQL表，我需要從中獲取結果，這些表是：

1. Towns
Fields Towncode and Townname

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

3. Phonebank
Fields student_id,contacted,towncode

現在，我需要一個mysql語句（a）從“學生”表中獲取家庭總數，以及（b）為該特定城鎮聯系的學生數。

到步驟（a）為止，我已經管理了哪個，並且速度非常快：

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 

但我無法設法插入另一個“ SELECT STATEMENT”來獲取該特定城鎮的電話數量。

你能幫忙嗎？

Answer 1

對城鎮進行單項記錄可能會傷害您。 我會先查詢學生表，然后再使用連接到城鎮的表。

對於索引，我將具有以下索引

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

假設“呼叫”數是PhoneBank表中具有特定城鎮代碼的行數，則可以添加另一個子選擇：

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 ;

您的問題有點含糊。 您可能希望在第二個查詢中使用count(distinct studentid) ，而不是count(*) 。

要優化此查詢，請創建以下索引：

• 城鎮（區號）
• 學生（區號，student_hh）
• 電話銀行（區號）