最佳SQLite查询删除重复项

Question

我有两个表,具有以下设置:

category: (id, name)
item: (id, name, category_id) - category_id is foreign key to category table

现在我正在编写一个查询来检索仅使用类别的类别表中的子集:

SELECT c.id, c.name
FROM   category c
WHERE  c.id IN (SELECT DISTINCT category_id FROM item)

以上查询工作正常.我只是想知道这是否是进行查询的最佳方式,或者是否还有其他我可以通过连接或其他方式做的事情

Answer 1

转换IN (SELECT)为EXISTS (SELECT ... WHERE )可能有助于:

SELECT c.id, c.name
FROM   category c
WHERE EXISTS (SELECT 1 FROM item WHERE item.category_id = c.id)

另一种可能性(我希望它更慢,但它总是取决于你的数据库):

SELECT c.id, c.name
FROM   category c
INNER JOIN item ON item.category_id = c.id
GROUP BY c.id

或者您可以使用DISTINCT而不是GROUP BY:

SELECT DISTINCT c.id, c.name
FROM   category c
INNER JOIN item ON item.category_id = c.id

如果速度非常重要,请不要忘记ANALYZE不时打电话:

http://www.sqlite.org/lang_analyze.html

其他一些有趣的变种:

SELECT c.id, c.name
FROM   category c
INNER JOIN (SELECT DISTINCT item.category_id ) AS i_c ON i_c.category_id = c.id

另一个:

SELECT c.id, c.name
FROM   category c

EXCEPT

SELECT c.id, c.name
FROM   category c
LEFT JOIN item ON item.category_id = c.id
WHERE item.category_id IS NULL