Question

I am joining between 3 tables and getting the wrong result. The goal is to list ALL the restaurants in the restaurants table and display the rating for any restaurant if the rating exists, otherwise display null, and only for ratings for burger.

This is the SQL:

    SELECT r.RestaurantID, RestaurantName, cr.Rating FROM Restaurant r
      LEFT JOIN CustRating cr ON cr.RestaurantID = r.RestaurantID
      LEFT JOIN FoodType ft ON ft.FoodTypeID = cr.FoodTypeID AND 
        ft.FoodTypeName = 'Burger'

This is the result:

However ‘Cafe C’ should have Rating = null because I only want to display ratings for Burgers. What’s the proper SQL?

The SQL Statements to create the tables and populate with data:

CREATE TABLE [dbo].[Restaurant](
    [RestaurantID] [int]  NOT NULL,
    [RestaurantName] [varchar](250) NOT NULL

)

CREATE TABLE [dbo].[FoodType](
    [FoodTypeID] [int]  NOT NULL,
    [FoodTypeName] [varchar](50) NOT NULL

)
CREATE TABLE [dbo].[CustRating](
    [RestaurantID] [int] NOT NULL,
    [FoodTypeID] [int] NOT NULL,
    [Rating] [smallint] NOT NULL
 ) 

 BEGIN TRANSACTION;
INSERT INTO [dbo].[CustRating]([RestaurantID], [FoodTypeID], [Rating])
SELECT 2, 1, 3 UNION ALL
SELECT 3, 2, 2
COMMIT;
GO

BEGIN TRANSACTION;
INSERT INTO [dbo].[FoodType]([FoodTypeID], [FoodTypeName])
SELECT 1, N'Burger' UNION ALL
SELECT 2, N'Taco'
COMMIT;
GO

BEGIN TRANSACTION;
INSERT INTO [dbo].[Restaurant]([RestaurantID], [RestaurantName])
SELECT 1, N'Cafe A' UNION ALL
SELECT 2, N'Cafe B' UNION ALL
SELECT 3, N'Cafe C'
COMMIT;
GO

Answer 1

SELECT              
                    Restaurant.*
                    ,CASE  WHEN FoodType.FoodTypeID IS NULL THEN NULL ELSE CustRating.Rating END AS Rating

 FROM               Restaurant Restaurant
 LEFT OUTER JOIN    CustRating CustRating 
 ON                 Restaurant.RestaurantID =   CustRating.RestaurantID
 LEFT OUTER JOIN    FoodType FoodType 
 ON                 FoodType.FoodTypeID = CustRating.FoodTypeID
 AND                FoodType.FoodTypeName = 'Burger'