mysqli：爲IN ID中的每一個選擇查詢和限制

Question

我有contacts具有id | city_id | category_id |電子郵件字段的表。
基本的查詢是

" 
SELECT id 
    , email 
    FROM contacts 
WHERE city_id IN (".implode(',', $arrCity).") 
    AND category_id IN (".implode(',', $arrCategory).") 
" 

但我需要設置一個限制：10排在城市的各個類別。可能嗎？

Answer 1

這很棘手。在MySQL中，最簡單的方法是使用變量：

SELECT id, email 
FROM (SELECT c.*, 
      (@rn := IF(@cc = CONCAT_WS(':', city_id, category_id), @rn + 1, 
         IF(@cc := CONCAT_WS(':', city_id, category_id), 1, 1) 
         ) 
      ) as rn 
     FROM (SELECT c.* 
      FROM contacts c 
      WHERE city_id IN (".implode(',', $arrCity).") AND 
        category_id IN (".implode(',', $arrCategory).") 
      ORDER BY city_id, category_id 
      ) c CROSS JOIN 
      (SELECT @cc := '', @rn := 0) params 
    ) c 
WHERE rn <= 10; 

這枚舉了每個城市/類別組合中的行。外WHERE限制在每個組合10。