A join is used to combine columns from two or more tables into a single result set. To join data from two tables you write the names of two tables in the FROM clause along with JOIN keyword and an ON phrase that specifies the join condition. The join condition indicates how two tables should be compared. In most cases they are compares on the base on the relationship of primary key of the first table and foreign key of the second table. In this article I will tell you about three important joins.
2. R ight Outer Join
3. Full outer join
1. Inner Join
2. Outer Join
I have two tables - Vendor table and Advance table. This is how my database tables and data looks like. I will be using these tables in my samples below.
Vendor table:
Figure 1.
Advance table:
Figure 2.
Now we are going to apply joins on these tables and see the data results.
Inner Joins
An inner join requires each record in the two joined tables to have a matching record. An inner join essentially combines the records from two tables (A and B) based on a given join-predicate. The result of the join can be defined as the outcome of first taking the Cartesian product (or cross-join) of all records in the tables (combining every record in table A with every record in table B) - then return all records which satisfy the join predicate. Actual SQL implementations will normally use other approaches where possible, since computing the Cartesian product is not very efficient. This type of join occurs most commonly in applications, and represents the default join-type.
Example: This is explicit inner join:
Use Vendor
GO
SELECT v.VendorId, v.VendorFName, v.VendorLName, a.royality, a.advance
FROM dbo.Vendor as v
INNER JOIN advance as a
ON v.VendorId = a.VendorId
WHERE v.VendorId <= 5
GO
Output:
Figure 3.
Example: This is implicit inner join:
Use Vendor
GO
SELECT * FROM Vendor, advance
WHERE Vendor.VendorId = advance.VendorId AND Vendor.VendorId <= 5
GO
Output:
Figure 4.
Type of inner joins
1. Equi-Join
An equi-join, also known as an equijoin, is a specific type of comparator-based join, or theta join that uses only equality comparisons in the join-predicate. Using other comparison operators (such as <) disqualifies a join as an equi-join. The query shown above has already provided an example of an equi-join:
Example:
Use Vendor
GO
SELECT * FROM Vendor INNER JOIN advance
ON Vendor.VendorId = advance.VendorId
GO
2. Natural Join
A natural join offers a further specialization of equi-joins. The join predicate arises implicitly by comparing all columns in both tables that have the same column-name in the joined tables. The resulting joined table contains only one column for each pair of equally-named columns.
Example:
Use Vendor
GO
SELECT * FROM Vendor NATURAL JOIN advance
GO
3. Cross Join
A cross join, Cartesian join or product provides the foundation upon which all types of inner joins operate. A cross join returns the Cartesian product of the sets of records from the two joined tables. Thus, it equates to an inner join where the join-condition always evaluates to True or join-condition is absent in statement.
Example:
Use Vendor
GO
SELECT * FROM Vendor CROSS JOIN advance
GO
Use Vendor
GO
SELECT * FROM Vendor, advance
GO
Outer Joins
An outer join retrieves all rows that satisfy the join condition plus unmatched rows in one or both tables. In most cases you use the equal operator to retrieve rows with matching columns. However you can also use any of the other comparison operators. When row with unmatched columns is retrieved any columns from the other table that are included in the result are given null values.
Note1: The OUTER keyword is optional and typically omitted
Note2: You can also code left outer joins and right outer joins using the implicit syntax.
Three types of outer joins.
1. Left Outer Join
The result of a left outer join (or simply left join) for tables A and B always contains all records of the "left" table (A), even if the join-condition does not find any matching record in the "right" table (B). This means that if the ON clause matches 0 (zero) records in B, the join will still return a row in the result but with NULL in each column from B. This means that a left outer join returns all the values from the left table, plus matched values from the right table (or NULL in case of no matching join predicate).
Example:
Use Vendor
GO
SELECT VendorFName, Vendor.VendorId, VendorLName, Advance
FROM Vendor LEFT JOIN advance
ON Vendor.VendorId = advance.VendorId
GO
Output:
Figure 5.
A right outer join (or right join) closely resembles a left outer join, except with the tables reversed. Every record from the "right" table (B) will appear in the joined table at least once. If no matching row from the "left" table (A) exists, NULL will appear in columns from A for those records that have no match in A.
Example :
Use Vendor
GO
SELECT VendorFName, advance.VendorId, VendorLName, Advance
FROM Vendor RIGHT JOIN advance
ON Vendor.VendorId = advance.VendorId
GO
A full outer join combines the results of both left and right outer joins. The joined table will contain all records from both tables, and fill in NULLs for missing matches on either side.
Example:
Use Vendor
GO
SELECT * FROM Vendor FULL OUTER JOIN advance
ON Vendor.VendorId = advance.VendorId
GO
OUTPUT:
Figure 6.
