如何在 MySQL 中進行分組和計數？

Question

我正在嘗試編寫一個查詢來提取發送給他們朋友的未刪除消息的總數。 這是表結構的屏幕截圖：

我試圖實現的輸出結果如下：

 id          messages_count
 ---------------------------
 1            4562
 2            7255

這是我到目前為止嘗試過的查詢：

SELECT u.id, count( CASE WHEN m.is_deleted = false THEN m.is_deleted END ) as messages_count
FROM users u
inner join messages m on u.id = m.to_id
GROUP BY u.id
ORDER BY u.id;

Answer 1

您是否嘗試過類似的方法？

SELECT u.id, count( m ) as messages_count
FROM users u
inner join messages m on u.id = m.to_id and m.is_deleted = false
GROUP BY u.id
ORDER BY u.id;

Answer 2

在WHERE子句中使用IS關鍵字。 嘗試這個：

SELECT u.id, count( m.id ) as messages_count
FROM users u
INNER JOIN messages m on u.id = m.to_id
WHERE m.is_deleted is FALSE
GROUP BY u.id
ORDER BY u.id;

Answer 3

除了傑克懷特的回答，你也可以這樣使用 sum ：

SELECT u.id,
   sum( CASE WHEN m.is_deleted = false THEN 1 else 0 END ) as messages_count
FROM users u
inner join messages m on u.id = m.to_id
GROUP BY u.id
ORDER BY u.id;

應該很清楚——如果不刪除，貢獻總和，否則不。