MYSQL：仅选择最新记录（在左侧联接表上）

Question

I have 2 tables:

Table1:

ID | Mobile Number | Name | Ordered Product| Order Date

Table2:

ID(foreign_key can be inserted multipletimes in this table) |Contacted_for | Time(timestamp)

I need a query to display all the data in Table1 and if the ID is present in Table 2, I need to display the last inserted record on Table2(with time) of that ID

My query is

select a.* , b.* FROM table1 a LEFT JOIN table2 b ON a.ID=b.ID GROUP BY a.ID ORDER BY b.Time DESC

Here in my query when I remove Group By a.ID, it works but shows all results. But I want to show final record of table2 only(no duplicate ID records)

Thanks in advance

Answer 1

You'll need some subquery's for that:

SELECT
    a.*, b.*
FROM
    table1 a
LEFT JOIN
    (SELECT c.id, d.contacted_for, c.time
     FROM
         (SELECT
            id,
            MAX(time) time
         FROM
             table2
         GROUP BY id
         ) c
     JOIN
         table2 d
         ON c.id = d.id AND d.time = c.time
     ) b
     ON a.id = b.id

Answer 2

You can also try this way.

Select * from table1 a left join
(Select * from table2 where id in (select max(id) from table2 group by id) ) b on a.id=b.id

Answer 3

The best way to do it is to have CreatedAt and ModifiedAt fields in every table in database. Then you just add ORDER BY CreatedAd LIMIT 1 . Not sure if your Time is what I mean.

What you also have is ID. Now, if ID is AutoIncremental then job should be easy. Use ORDER BY id DESC LIMIT 1

Answer 4

Try this ... I hope it may works

select a.* , b1.* 
FROM table1 a 
LEFT JOIN table2 b1
ON ( a.ID=b1.ID)  
LEFT JOIN table2 b2
ON ( a.ID=b1.ID)  AND 
(b1.Time < b2.Time OR b1.Time = b2.Time AND b1.ID < b2.ID)
WHERE b2.ID IS NULL
GROUP BY a.ID ORDER BY b1.Time DESC