Joining tables

578 views

Published on

0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total views
578
On SlideShare
0
From Embeds
0
Number of Embeds
2
Actions
Shares
0
Downloads
9
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide

Joining tables

  1. 1. Home Contents Joining tables In this part of the SQLite tutorial, we will join tables in SQLite. The real power and benefits from relational databases come from joining tables. The SQL JOIN clause combines records from two or more tables in a database. There are basically two types of joins. INNER and OUTER. In this part of the tutorial, we will work with Customers and Reservations tables. sqlite> SELECT * FROM Customers; CustomerId Name ---------- ----------- 1 Paul Novak 2 Terry Neils 3 Jack Fonda 4 Tom Willis Values from the Customers table. sqlite> SELECT * FROM Reservations; Id CustomerId Day
  2. 2. -- ---------- ---------- 1 1 2009-22-11 2 2 2009-28-11 3 2 2009-29-11 4 1 2009-29-11 5 3 2009-02-12 Values from the Reservations tables. Inner joins The inner join is the most common type of joins. It is the default join also. The inner join selects only those records from database tables that have matching values. We have three types of INNER JOINS. INNER JOIN, NATURAL INNER JOIN and CROSS INNER JOIN. The INNER keyword can be omitted. INNER JOIN sqlite> SELECT Name, Day FROM Customers AS C JOIN Reservations AS R ON C.CustomerId=R.CustomerId; Name Day ----------- ---------- Paul Novak 2009-22-11
  3. 3. Terry Neils 2009-28-11 Terry Neils 2009-29-11 Paul Novak 2009-29-11 Jack Fonda 2009-02-12 In this SELECT statement, we have selected all customers, that have made some reservations. Note, that we have omitted the INNER keyword. The statement is equivalent to the following one: sqlite> SELECT Name, Day FROM Customers, Reservations WHERE Customers.CustomerId = Reservations.CustomerId; Name Day ----------- ---------- Paul Novak 2009-22-11 Terry Neils 2009-28-11 Terry Neils 2009-29-11 Paul Novak 2009-29-11
  4. 4. Jack Fonda 2009-02-12 We get the same data. NATURAL INNER JOIN The NATURAL INNER JOIN automatically uses all the matching column names for the join. In our tables, we have a column named CustomerId in both tables. sqlite> SELECT Name, Day FROM Customers NATURAL JOIN Reservations; Name Day ----------- ---------- Paul Novak 2009-22-11 Terry Neils 2009-28-11 Terry Neils 2009-29-11 Paul Novak 2009-29-11 Jack Fonda 2009-02-12 We get the same data. The SQL statement is less verbose. CROSS INNER JOIN The CROSS INNER JOIN combines all records from one table with all records from another table. This type of join has little practical value. It is also called a cartesian product of records.
  5. 5. sqlite> SELECT Name, Day FROM Customers CROSS JOIN Reservations; Name Day ----------- ---------- Paul Novak 2009-22-11 Paul Novak 2009-28-11 Paul Novak 2009-29-11 Paul Novak 2009-29-11 Paul Novak 2009-02-12 Terry Neils 2009-22-11 Terry Neils 2009-28-11 Terry Neils 2009-29-11 Terry Neils 2009-29-11 Terry Neils 2009-02-12 ... The same result can be achieved with the following SQL statement:
  6. 6. SELECT Name, Day FROM Customers, Reservations; Outer joins An outer join does not require each record in the two joined tables to have a matching record. There are three types of outer joins. Left outer joins, right outer joins, and full outer joins. SQLite only supports left outer joins. LEFT OUTER JOIN The LEFT OUTER JOIN returns all values from the left table, even if there is no match with the right table. It such rows, there will be NULL values. In other words, left outer join returns all the values from the left table, plus matched values from the right table. Note, that the OUTER keyword can be omitted. sqlite> SELECT Name, Day FROM Customers LEFT JOIN Reservations ON Customers.CustomerID=Reservations.CustomerId; Name Day ----------- ---------- Paul Novak 2009-22-11 Paul Novak 2009-29-11 Terry Neils 2009-28-11 Terry Neils 2009-29-11 Jack Fonda 2009-02-12
  7. 7. Tom Willis NULL Here we have all customers with their reservations, plus a customer, who has no reservation. There is NULL value in his row. We can use the USING keyword to achieve the same result. The SQL statement will be less verbose. sqlite> SELECT Name, Day FROM Customers LEFT JOIN Reservations USING (CustomerId); Name Day ----------- ---------- Paul Novak 2009-22-11 Paul Novak 2009-29-11 Terry Neils 2009-28-11 Terry Neils 2009-29-11 Jack Fonda 2009-02-12 Tom Willis NULL Same result, with shorter SQL statement. NATURAL LEFT OUTER JOIN The NATURAL LEFT OUTER JOIN automatically uses all the matching column names for the join. sqlite> SELECT Name, Day FROM Customers NATURAL LEFT OUTER JOIN Reservations;
  8. 8. Name Day ----------- ---------- Paul Novak 2009-22-11 Paul Novak 2009-29-11 Terry Neils 2009-28-11 Terry Neils 2009-29-11 Jack Fonda 2009-02-12 Tom Willis NULL Same result, but with fewer key strokes. In this part of the SQLite tutorial, we were joining tables.

×