Sql why use join

You can execute the code below to create and populate data into these two tables. All this data is hypothetical and you can create in any of your existing databases. CompanyID in this table is the foreign key that is referencing to the Primary key of the PizzaCompany table created above. The following table shows data in the Foods table. This table stores information like units sold per food item and also the pizza outlet CompanyId that delivers it.

Now, if we would like to see the items and also the units sold by each pizza company, we can combine these two tables with the help of an inner join clause being used on the field CompanyId in our case this shares a foreign key relationship. For each row in the table PizzaCompany, Inner Join compares and finds the matching rows in the table Foods and returns all the matching rows as shown below. With the help of the above result set, we can make out the items and also the count of items delivered by pizza outlets in various cities.

I am going to quickly create a table WaterPark and load some arbitrary data into it as shown below. As the saying goes, the picture is worth a thousand words. Now we are going to include this third table in the SQL Inner Join clause to see how it is going to impact the result set. Execute the code below to see all the food distribution across the waterparks by the Pizza outlets. As shown below, with the addition of inner join on WaterPark, CompanyId 6,7 apart from 5 are also excluded from the final result set as the condition w.

CompanyId is not satisfied for Ids 6,7. This is how SQL Inner join helps to return specific rows of data from multiple tables. Assume that we would like to get the rows where units sold were more than 6. In the following query, the WHERE clause is added to extract results with value more than 6 for units sold. Execute above code in SSMS to see the below result.

Four such records are returned by this query. SQL Inner Join permits us to use Group by clause along with aggregate functions to group the result set by one or more columns. The result is NULL in the consecutive tables when there is no match.

In this case, we would make the User Table the first left table to use for the left join. For a more Detailed look at the Left Join click here. In addition to these common join types, there are some methods which will result in additional rows in your output table as well as more columns.

Two of these join types are called Union and Cross Join. A Union Join will stack tables on top of each other resulting in new rows. For a more detailed look at the Union Join click here. This can result in enormous tables and should be used with caution. For a more detailed look at the Cross Join click here. Cross Joins will likely only be used when your tables contain single values that you want to join together without a common dimension.

A JOIN clause is used to combine rows from two or more tables, based on a related column between them. The relationship between the two tables above is the "CustomerID" column. Insert the missing parts in the JOIN clause to join the two tables Orders and Customers , using the CustomerID field in both tables as the relationship between the two tables.

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