在mysql中使用右表的条件左连接

Question

I have been trying to wrap my head around this issue but I am not making much progress. My goal is to do a left join between two tables with criteria for the right table. I would like to see the list of all products and prices for the current day even though there is no pricing row for the current day. Here is an example of the code:

SELECT products.id, products.name, prices.price
FROM products LEFT JOIN prices
ON products.id = prices.id
WHERE prices.date = CURRENT_DATE

This produces only the products with price information for the current date.

OK, so that is just the first part of my issue. I would eventually like to pull the price for CURRENT_DATE + INTERVAL 1 DAY as well.

Any information would be greatly appreciated.

Answer 1

Assuming PRICES.date is a DATETIME data type, use:

   SELECT pd.id, 
          pd.name, 
          pr.price
     FROM PRODUCTS pd
LEFT JOIN PRICES pr ON pr.id = pd.id
                   AND DATE(pr.date) = CURRENT_DATE

I used the DATE function to remove the time portion, because CURRENT_DATE won't include the time portion while DATETIME records will.

In this example, the date criteria is being applied before the JOIN is made. It's like a derived table, filtering down the information before the JOIN is made -- which'll produce different results than if the criteria was specified in the WHERE clause.

To get the list of products and prices for tomorrow, use:

   SELECT pd.id, 
          pd.name, 
          pr.price
     FROM PRODUCTS pd
LEFT JOIN PRICES pr ON pr.id = pd.id
                   AND DATE(pr.date) = DATE_ADD(CURRENT_DATE, INTERVAL 1 DAY)

Reference:

• DATE_ADD

If you want both todays and tomorrows prices in a single query, use:

   SELECT pd.id, 
          pd.name, 
          pr1.price AS price_today,
          pr2.price AS price_tomorrow
     FROM PRODUCTS pd
LEFT JOIN PRICES pr1 ON pr1.id = pd.id
                   AND DATE(pr1.date) = CURRENT_DATE
LEFT JOIN PRICES pr2 ON pr2.id = pd.id
                   AND DATE(pr2.date) = DATE_ADD(CURRENT_DATE, INTERVAL 1 DAY)

Answer 2

Strong answers above. Adding to OMG Ponies' reply... having 'right' table criteria in the original WHERE statement basically turns the LEFT OUTER JOIN into an INNER JOIN. Just a different way of looking at it that might help.

Answer 3

SELECT id,
       name,
       (SELECT price
          FROM prices
         WHERE id = products.id
           AND prices.date = CURRENT_DATE
       ) AS price,
       (SELECT price
          FROM prices
         WHERE id = products.id
           AND prices.date = DATE_ADD(CURRENT_DATE, INTERVAL 1 DAY)
       ) AS price_tomorrow
  FROM products