Sql_ Query the current status of each tid: that is, the latest publication of the category

First, the problem scenario

1. A table has a key field tid,action,dateline, etc., the TID represents the id,action of the Post, which represents the status of the post being manipulated, Dateline represents the timestamp of the data being inserted;

2. In this table, each TID will have several different Dateline inserted details, the last time the action field in the detail inserted indicates the status of the current Tid's post;

3. Now you want to query the current status of each tid, that is, to query the latest published record

4. Sample Data in table:

   
   

Second, SQL statement:

Select A.* from Pre_forum_threadmod as A, (select Tid,max (Dateline) as Dateline from Pre_forum_threadmod Group by Tid) as B where A.tid=b.tid and A.dateline=b.dateline

Third, SQL statement parsing:

1. In the above SQL statements, the largest dateline of each TID is identified, and the query generates a temporary table B;

   (select Tid,max (Dateline) as Dateline from Pre_forum_threadmod Group by Tid) as B

2. Then the original table A and temporary Table B Association query can:

   where A.tid=b.tid and A.dateline=b.dateline

3. After enquiry, for example, the latest record of each TID can be found: