Joins in SQL Server allows the retrieval of data records from one or more tables having some relation between them. Logical operators can also be used to drill down the number of records to get the desired output from sql join queries.
Inner Join: Inner Join is a default type join of SQL Server. It uses logical operators such as =, >,< to match the records in two tables. Inner Join includes equi join and natural joins.
SQL Inner Join Examples
SELECT C.CATEGORYID, C.CATEGORYNAME, P.PRODUCTID, P.PRODUCTNAME, P.UNITPRICE FROM CATEGORIES C INNER JOINPRODUCTS P ON P.CATEGORYID = C.CATEGORYIDWHERE P.UNITPRICE = 10ORDER BY C.CATEGORYNAME, P.PRODUCTNAME
SQL Inner Natural
This inner join query will return the categoryid, categoryname, productid, productname, unitprice where product unit price = 10
SQL Inner Natural Join Examples
SELECT C.*, P.PRODUCTID, P.PRODUCTNAME FROM CATEGORIES C INNER JOINPRODUCTS P ON P.CATEGORYID = C.CATEGORYID
This natural join query will return all the columns of categories table and prodcutId and productName from products table. You can further modify this natural inner join query as per your requirements to visualize the data by specifying the column names of categories table also.
SQL Inner Equi Join Examples
Inner join is a default type of SQL Join that return the records matching in all the tables joined in sql query satisfying the condition specified in WHERE clause.
Inner join includes 3 types of joins similar to one another.
Self Join: Self join joins a single sql database table to itself. Equi Join: Equi Join returns all the columns from both tables and filters the records satisfying the matching condition specified in Join “ON” statement of sql inner join query.
USE NORTHWIND SELECT * FROM CATEGORIES C INNER JOINPRODUCTS P ON P.CATEGORYID = C.CATEGORYID
Result will display the following columns: CategoryID, CategoryName, Description, Picture, ProductID, ProductName, SupplierID, CategoryID, QuantityPerUnit, UnitPrice, UnitsInStock, UnitsOnOrder, ReorderLevel, Discontinued Above equi join sql query will display the categoryId two times in a row because both the tables have categoryId column. You can convert the result into natural join by elimination the identical columns and unnecessary columns.
Outer Join: Outer Join has further 3 sub categories as left, right and full. Outer Join uses these category names as keywords that can be specified in the FROM clause. Left Outer Join: Left Outer Join returns all the rows from the table specified first in the Left Outer Join Clause. If in the left table any row has no matching record in the right side table then that row returns null column values for that particular tuple. Inner joins return only those rows from both sql database tables having matching records in both the tables whereas left outer join returns all the rows from the left table and related matching records from the other one.
SQL Left Outer Join Example:
SELECT A.AU_FNAME, A.AU_LNAME, P.PUB_NAMEFROM AUTHORS A LEFT OUTER JOIN PUBLISHERS PON A.CITY = P.CITYORDER BY A.AU_LNAME, A.AU_FNAME
This left outer join query retrieves the author names and publisher name having same cities. Here all rows retrieved from the left table i.e. authors and publisher name having the similar city other columns of pub_name column are null due to no match found in the right table. Right Outer Join: Right Outer Join is exactly the reverse method of Left Outer Join. It returns all the rows from right table and returns null values for the rows having no match in the left joined table. Just change the left keyword to right outer join in above example; you will get the reverse output of left outer join in the form of right outer join.
SQL Right Outer Join query Example:
SELECT A.AU_FNAME, A.AU_LNAME, P.PUB_NAMEFROM AUTHORS A RIGHT OUTER JOIN PUBLISHERS PON A.CITY = P.CITYORDER BY A.AU_LNAME, A.AU_FNAME
Full Outer Join: Full outer join returns all the rows from both left and right joined tables. If there is any match missing from the left table then it returns null column values for left side table and if there is any match missing from right table then it returns null value columns for the right side table. To retrieve all the records from left as well as right table unless the records have matching relations in each row you can use SQL FULL OUTER JOIN. You can consider the examples of last two articles about left outer join and right outer join, in which left outer join retrieves all records from the left table and as all records of right table in right outer join along with null values for the columns having no matching records in any tuple. To retain all the records of left as well as right table along with null values for non matching rows displaying the combination of results of left outer and right outer join, FULL OUTER JOIN is the best solution.
SQL FULL outer join example:
SELECT A.AU_FNAME, A.AU_LNAME, P.PUB_NAMEFROM AUTHORS A FULL OUTER JOIN PUBLISHERS PON A.CITY = P.CITYORDER BY A.AU_LNAME, A.AU_FNAME
Cross Join: Cross join works as a Cartesian product of rows for both left and right table. It combined each row of left table with all the rows of right table. SQL Cross join returns the output result as a Cartesian product of both database tables. Let left table has 10 rows and right table has 8 rows then SQL CROSS Join will return 180 rows combining each record of left table with all records of right side table.
Consider the following example of CROSS Join:
USE PUBSSELECT AU_FNAME, AU_LNAME, PUB_NAMEFROM AUTHORS CROSS JOIN PUBLISHERS ORDER BY AU_FNAME
Above cross join will return 23 * 8 = 184 results by multiplying each row of authors table with publishers table. SQL CROSS Join with WHERE clause By just adding the where clause with Cross join sql query it turns the output result into inner join.
