Question

我有这个查询，它正确显示结果。我没有使用join，但它似乎仍然返回正确的值，这怎么可能？

SELECT connections.id,
       connections.word_id,
       connections.order_id,
       connections.top,
       connections.deleted,
       (UNIX_TIMESTAMP() - UNIX_TIMESTAMP(connections.modified)) AS modified_ago,
       words_en.word,
       (CASE WHEN words_en.user_id = 1 THEN "You" ELSE words_en.user_id END) AS created_by
FROM connections, words_en
WHERE connections.word_id = words_en.id AND connections.user_id = 1
IN (SELECT COUNT(*) connections WHERE /* Result of previous select connections.word_id */ AND connections.user_id != 1)
ORDER BY connections.order_id

这样做是否足够：connections.word_id = words_en.id加入？
我还想为此添加子查询。这是什么语法？

子查询应计算conenctions.word_id在连接表中显示的次数connections.user_id != 1 ergo其他用户使用此单词的次数。

我还需要另一个加入users.name_surname的{{1}}。

Answer 1

要连接我们可以使用的表，（逗号）或连接关键字。如果我们使用，（逗号）它将根据我们的条件给出确切的结果。如果我们使用join关键字，我们可以使用许多类型的连接，如左连接右连接等等。这些连接将产生不同的结果。

我们可以说，（逗号）连接与JOIN相同但不是LEFT JOIN。

以下是编写子查询的示例

SELECT connections.id,
       connections.word_id,
       connections.order_id,
       connections.top,
       connections.deleted,
       (UNIX_TIMESTAMP() - UNIX_TIMESTAMP(connections.modified)) AS modified_ago,
       words_en.word,
       words_en.user_id 
FROM connections, words_en
WHERE connections.word_id = words_en.id AND connections.user_id IN (select id from connections)
ORDER BY connections.order_id

select id from connections是子查询

第3点尝试以下

SELECT users.name_surname 
FROM users, words_en
WHERE users.user_id = words_en.user_id
ORDER BY users.user_id

Answer 2

用于连接操作：

SELECT connections.id,
   connections.word_id,
   connections.order_id,
   connections.top,
   connections.deleted,
   (UNIX_TIMESTAMP() - UNIX_TIMESTAMP(connections.modified)) AS modified_ago,
   words_en.word,
   words_en.user_id 
FROM connections
LEFT JOIN  words_en ON connections.word_id = words_en.id AND connections.user_id = 1
WHERE 
     words_en.id IS NOT NULL
ORDER BY connections.order_id

mysql查询（join）和添加子查询

2 个答案: