Graphical Joins in More Detail

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1 Graphical Joins in More Detail Using the Connector, data is made available through the addition of containers and relevant expressions. The source of the underlying data can be a Table, a View, a Stored Procedure, SQL Query or a user defined SQL Join or a Graphical Join. To create a container based on a SQL Join, you should add a container, selecting the option SQL Join from the Select Container Type window and providing a name for the container. Before adding expressions, you would then type in the join syntax in the Source Container (Join) field. The source should thus be the FROM clause (excluding the FROM keyword) of a SQL query that would define the join. The syntax of the Join can be verified by using the Check/Test facility for the container. When the SQL join is verified to be correct, the Connector module can select to Add Expressions. The expression list will then include all fields from all the tables in the Join. There are two main SQL Join styles that are used by database systems. For some systems either will work, while for others only one of the methods will work. It is recommended where possible to use Syntax 2 (the newer ANSI style Join). Join Syntax 1 TABLE1, TABLE2, TABLE3 WHERE TABLE1.KEYFIELD = TABLE2.KEYFIELD AND TABLE2.KEYFIELD_2 = TABLE3.KEYFIELD Example timesheet, employee, department WHERE timesheet.employee_id = employee.id AND employee.department_id = department.id Join Syntax 2 TABLE1 [INNER LEFT RIGHT] JOIN TABLE2 ON TABLE1.KEYFIELD = TABLE2.KEYFIELD [INNER LEFT RIGHT] JOIN TABLE3 AND TABLE2.KEYFIELD_2 = TABLE3.KEYFIELD Example timesheet INNER JOIN employee ON timesheet.employee_id = employee.id INNER JOIN department ON employee.department_id = department.id INNER JOIN ON TIP: Some systems allow tables to have names, which include space characters. Where this is the case, it is necessary to surround table names and field names with square brackets.

2 Understanding Inner, Outer or Full Outer Joins An Inner Join is created by default, which can be changed by right-clicking on the join box on either side of the join and selecting Outer join. The reverse polarity option on the shortcut menu switches the join between a Left and Right Outer Join. Inner Join An Inner Join returns all rows from both tables where there is a match. If there are rows in Table 1 that do not have matches in Table 2, those rows will NOT be listed. This is the simplest, most understood Join and is the most common. This query will return all of the records in the left table (table A) that have a matching record in the right table (table B) Example: P_ID LastName FirstName Region O_ID OrderNo P_ID To create a list with all the customers with any orders: SELECT Customers.Lastname,Customers.FirstName,Orders.OrderNo from Customer INNER JOIN Orders ON Customers.P_Id=Orders.P_Id

3 ORDER BY Customers.Lastname Result of above: LastName FirstName OrderNo Full Outer Join 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. This Join can also be referred to as a FULL OUTER JOIN or a FULL JOIN. This query will return all of the records from both tables, joining records from the left table (table A) that match records from the right table (table B). Example: P_ID LastName FirstName Region O_Id OrderNo P_Id

4 To list all the customers and their orders, and all the orders with their customers, the SQL script will be: SELECT Customers.LastName, Customers.FirstName, Orders.OrderNo from Customers FULL JOIN Orders ON Customers.P.Id=Orders.P_Id ORDER BY Customers.LastName Result of above: LastName FirstName OrderNo Teller Tammy Outer Join An Outer join can be either a Left Outer or a Right Outer join. Left Outer Join Left outer join produces a complete set of records from Table A, with the matching records (where available) in Table B. If there is no match, the right side will contain null. Example: P_ID LastName FirstName Region

5 O_Id OrderNo P_Id To list all the Customers and their orders, the following SQL script can be used: SELECT Customers.Lastname, Customers.FirstName, Orders.OrderNo from Customers LEFT JOIN Orders ON Customers.P_Id=Orders.P_Id ORDER BY Customers.LastName Result of above: LastName FirstName OrderNo Teller Tammy Even though the Customer. Tammy Teller, has no order, she will still be returned. Right Outer Join A right outer join is much like a left outer join, except that the tables are reversed. A right outer join returns all the values from the right table even if there are no matches in the left table. This query will return all of the records in the right table (table B) regardless if any of those records have a match in the left table (table A). It will also return any matching records from the left table. Example

6 P_ID LastName FirstName Region O_Id OrderNo P_Id To list all the Orders with Customers, the following SQL script can be used: SELECT Customers.Lastname, Customers.FirstName, Orders.OrderNo from Customers RIGHT JOIN Orders ON Customers.P_Id=Orders.P_Id ORDER BY Customers.LastName Result: LastName FirstName OrderNo Primary and Secondary (Foreign) Keys When creating a relational database design, a unique key or primary key is used to identify each row in a table. A unique key or primary key is usually a single column or set of columns in a table in the database. No two rows in a table can have the same value (or combination of values) in this column. A unique key or primary key must uniquely identify all possible rows that exist in a table and not only the currently existing rows. Examples of unique keys are: Identity Document numbers (associated with a specific person) or

7 ISBNs (associated with a specific book). Employee Numbers Unique or primary keys can be referenced by secondary (foreign) keys. The secondary (foreign) key identifies a column in one table that refers to a column in another table. A table may have multiple secondary (foreign) keys. This can result in a single table having many other tables that are related to using the secondary (Foreign) key. Joining tables incorrectly by using the wrong secondary (foreign) key/primary key relationships often results in report writing errors. Examples of secondary (foreign) keys are: Transaction details for customer or product masterfile tables