From 612d266233787ca2e631801f9e520d6080453957 Mon Sep 17 00:00:00 2001 From: CyC2018 Date: Wed, 11 Dec 2019 00:26:43 +0800 Subject: [PATCH] auto commit --- docs/notes/Leetcode-Database 题解.md | 23 ++++++++++++++++------- notes/Leetcode-Database 题解.md | 23 ++++++++++++++++------- 2 files changed, 32 insertions(+), 14 deletions(-) diff --git a/docs/notes/Leetcode-Database 题解.md b/docs/notes/Leetcode-Database 题解.md index 64aed608..07010e22 100644 --- a/docs/notes/Leetcode-Database 题解.md +++ b/docs/notes/Leetcode-Database 题解.md @@ -230,7 +230,7 @@ https://leetcode.com/problems/classes-more-than-5-students/description/ ## Solution -对 class 列进行分组之后，再使用 count 汇总函数统计数量，统计之后使用 having 进行过滤。 +对 class 列进行分组之后，再使用 count 汇总函数统计每个分组的记录个数，之后使用 HAVING 进行筛选。HAVING 针对分组进行筛选，而 WHERE 针对每个记录（行）进行筛选。 ```sql SELECT @@ -293,7 +293,7 @@ https://leetcode.com/problems/duplicate-emails/description/ ## Solution -对 Email 进行分组，如果相同 Email 的数量大于等于 2，则表示该 Email 重复。 +对 Email 进行分组，如果并使用 COUNT 进行计数统计，结果大于等于 2 的表示 Email 重复。 ```sql SELECT @@ -354,7 +354,7 @@ https://leetcode.com/problems/delete-duplicate-emails/description/ 只保留相同 Email 中 Id 最小的那一个，然后删除其它的。 -连接： +连接查询： ```sql DELETE p1 @@ -373,7 +373,14 @@ DELETE FROM Person WHERE - id NOT IN ( SELECT id FROM ( SELECT min( id ) AS id FROM Person GROUP BY email ) AS m ); + id NOT IN ( + SELECT id + FROM ( + SELECT min( id ) AS id + FROM Person + GROUP BY email + ) AS m + ); ``` 应该注意的是上述解法额外嵌套了一个 SELECT 语句，如果不这么做，会出现错误：You can't specify target table 'Person' for update in FROM clause。以下演示了这种错误解法。 @@ -383,7 +390,11 @@ DELETE FROM Person WHERE - id NOT IN ( SELECT min( id ) AS id FROM Person GROUP BY email ); + id NOT IN ( + SELECT min( id ) AS id + FROM Person + GROUP BY email + ); ``` 参考：[pMySQL Error 1093 - Can't specify target table for update in FROM clause](https://stackoverflow.com/questions/45494/mysql-error-1093-cant-specify-target-table-for-update-in-from-clause) @@ -392,8 +403,6 @@ WHERE 与 182 相同。 - - # 175. Combine Two Tables https://leetcode.com/problems/combine-two-tables/description/ diff --git a/notes/Leetcode-Database 题解.md b/notes/Leetcode-Database 题解.md index 64aed608..07010e22 100644 --- a/notes/Leetcode-Database 题解.md +++ b/notes/Leetcode-Database 题解.md @@ -230,7 +230,7 @@ https://leetcode.com/problems/classes-more-than-5-students/description/ ## Solution -对 class 列进行分组之后，再使用 count 汇总函数统计数量，统计之后使用 having 进行过滤。 +对 class 列进行分组之后，再使用 count 汇总函数统计每个分组的记录个数，之后使用 HAVING 进行筛选。HAVING 针对分组进行筛选，而 WHERE 针对每个记录（行）进行筛选。 ```sql SELECT @@ -293,7 +293,7 @@ https://leetcode.com/problems/duplicate-emails/description/ ## Solution -对 Email 进行分组，如果相同 Email 的数量大于等于 2，则表示该 Email 重复。 +对 Email 进行分组，如果并使用 COUNT 进行计数统计，结果大于等于 2 的表示 Email 重复。 ```sql SELECT @@ -354,7 +354,7 @@ https://leetcode.com/problems/delete-duplicate-emails/description/ 只保留相同 Email 中 Id 最小的那一个，然后删除其它的。 -连接： +连接查询： ```sql DELETE p1 @@ -373,7 +373,14 @@ DELETE FROM Person WHERE - id NOT IN ( SELECT id FROM ( SELECT min( id ) AS id FROM Person GROUP BY email ) AS m ); + id NOT IN ( + SELECT id + FROM ( + SELECT min( id ) AS id + FROM Person + GROUP BY email + ) AS m + ); ``` 应该注意的是上述解法额外嵌套了一个 SELECT 语句，如果不这么做，会出现错误：You can't specify target table 'Person' for update in FROM clause。以下演示了这种错误解法。 @@ -383,7 +390,11 @@ DELETE FROM Person WHERE - id NOT IN ( SELECT min( id ) AS id FROM Person GROUP BY email ); + id NOT IN ( + SELECT min( id ) AS id + FROM Person + GROUP BY email + ); ``` 参考：[pMySQL Error 1093 - Can't specify target table for update in FROM clause](https://stackoverflow.com/questions/45494/mysql-error-1093-cant-specify-target-table-for-update-in-from-clause) @@ -392,8 +403,6 @@ WHERE 与 182 相同。 - - # 175. Combine Two Tables https://leetcode.com/problems/combine-two-tables/description/