国外版

算法题（国外版）/find-followers-count.html

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+<p>Table: <code>Followers</code></p>
+
+<pre>
++-------------+------+
+| Column Name | Type |
++-------------+------+
+| user_id     | int  |
+| follower_id | int  |
++-------------+------+
+(user_id, follower_id) is the primary key for this table.
+This table contains the IDs of a user and a follower in a social media app where the follower follows the user.</pre>
+
+<p>&nbsp;</p>
+
+<p>Write an SQL query that will, for each user, return the number of followers.</p>
+
+<p>Return the result table ordered by <code>user_id</code>.</p>
+
+<p>The query result format is in the following example.</p>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> 
+Followers table:
++---------+-------------+
+| user_id | follower_id |
++---------+-------------+
+| 0       | 1           |
+| 1       | 0           |
+| 2       | 0           |
+| 2       | 1           |
++---------+-------------+
+<strong>Output:</strong> 
++---------+----------------+
+| user_id | followers_count|
++---------+----------------+
+| 0       | 1              |
+| 1       | 1              |
+| 2       | 2              |
++---------+----------------+
+<strong>Explanation:</strong> 
+The followers of 0 are {1}
+The followers of 1 are {0}
+The followers of 2 are {0,1}
+</pre>