MySQL中如何進行分組和計數？

Question

我正在嘗試編寫一個查詢，提取發送給朋友的未刪除訊息的總數。這是表結構的螢幕截圖：

我想要實現的輸出結果如下：

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

這是我迄今為止嘗試過的查詢：

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

你嘗試過類似的事情嗎？

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;