Question

我有以下数据库。每次对话都有两名参与者。我要计算的是session.id.participant_id =参与者ID1和参与者.participant_id =参与者ID2时的session_id的数量。

CREATE TABLE IF NOT EXISTS `users` (
    `user_id` BIGINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    `user_name` VARCHAR(16)
);

CREATE TABLE IF NOT EXISTS `conversation` (
    `conversation_id` BIGINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    `user_id` BIGINT UNSIGNED,
    `time_started` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    `time_closed` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    FOREIGN KEY (user_id) REFERENCES users(user_id)
);

CREATE TABLE IF NOT EXISTS `participant` (
    `id` BIGINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    `conversation_id` BIGINT UNSIGNED,
    `participant_id` BIGINT UNSIGNED,
    `time_joined` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    `time_left` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    FOREIGN KEY (conversation_id) REFERENCES conversation(conversation_id),
    FOREIGN KEY (participant_id) REFERENCES users(user_id)
);

我尝试了类似的方法，但最终计数为0：

SELECT count(conversation.conversation_id)
FROM `conversation`
INNER JOIN `participant` on participant.conversation_id = conversation.conversation_id
WHERE participant.participant_id = ? AND participant.participant_id = ?

我设法以不同的方式进行操作，但是我想知道是否可以在一个查询中进行操作。谢谢你的帮助！：-）

Answer 1

我目前无法检查，但是这样的方法应该可以工作：

SELECT count(c.conversation_id)
FROM conversation c
INNER JOIN participant p1 ON p1.conversation_id = c.conversation_id
INNER JOIN participant p2 ON p2.conversation_id = c.conversation_id
WHERE p1.participant_id = ? AND p2.participant_id = ?

具有两个相同条件的SELECT count（）

1 个答案: