在JOIN上获取SQL查询的唯一/不同结果

Question

I have 2 tables: products and reviews, where each product has many reviews:

table products has the following columns: id, name table reviews has the following columns: id, productid, created_on, rating, review. Where the productid is a foreign key, and created_on is type datetime.

Sample data as follows:

 <table> <tr> <th>product id</th> <th>product name</th> </tr> <tr> <td>1</td> <td>Foo</td> </tr> <tr> <td>2</td> <td>Bar</td> </tr> </table> <table> <tr> <th>review id</th> <th>product id</th> <th>rating</th> <th>review</th> <th>created_on</th> </tr> <tr> <td>1</td> <td>1</td> <td>5</td> <td>Perfect foo</td> <td>2017-1-1Z00:00:00</td> </tr> <tr> <td>2</td> <td>1</td> <td>1</td> <td>This foo is not the foo I was looking for</td> <td>2017-2-2Z00:00:00</td> </tr> <tr> <td>3</td> <td>1</td> <td>4</td> <td>Foo-tastic</td> <td>2017-3-3Z00:00:00</td> </tr> <tr> <td>4</td> <td>2</td> <td>4</td> <td>Bar is Bar/10</td> <td>2017-3-3Z00:00:00</td> </tr> <tr> <td>4</td> <td>2</td> <td>5</td> <td>Barmendous!!!</td> <td>2017-1-1Z00:00:00</td> </tr> </table> 

I want to be able to get the latest review for each product but I'm unsure how to do it. It should be something like:

SELECT products.product_name, reviews.rating, reviews.review FROM products LEFT JOIN products ON products.id = reviews.productid ORDER BY reviews.created_on DESC;

But this will return multiple results for each product. I only need one review for each product, preferably the most recent review.

MySQL 5.x or above is the preferred database in this case.

Sample out is as follows:

 <table> <tr> <th>product name</th> <th>rating</th> <th>review</th> </tr> <tr> <td>Foo</td> <td>4</td> <td>Footastic</td> </tr> <tr> <td>Bar</td> <td>4</td> <td>Bar is Bar/10</td> </tr> <table> 

Answer 1

If you want the latest review for each product, then use a WHERE clause:

SELECT p.product_name, r.*
FROM products p LEFT JOIN 
     reviews r
     ON p.id = r.productid AND
        r.created_on = (SELECT MAX(r2.created_on)
                        FROM reviews r2
                        WHERE r2.productid = r.productid
                       );

Answer 2

Do your query around the other way:

SELECT r.*, p.*
FROM reviews AS r
LEFT JOIN products AS p
    ON p.id = r.productid
GROUP BY r.productid 
ORDER BY r.date DESC;