Query:  Statement that allows data retrieval

            View:  A virtual table; a saved query (the SELECT statement, not the result)



SELECT statement (DML)


- retrieves a limited set of data from one or more tables using criteria specified in the WHERE clause

- often used to perform calculations on the data selected

- the result set is displayed as a table (columns and rows)


Single-table example (review): 


Current Product List:  all data comes from the Products table




            SELECT column list

            FROM tablename

            WHERE criteria

            ORDER BY column list


Select from two tables:  Example


Run the Orders Query (Orders Qry on the Query list):  It lists all orders for all customers, without going into line items (order details), by retrieving related data from the Orders and Customers tables.

orders query result set

Note the number of rows and columns; several columns are repeated more often than strictly necessary.


Use the drop-down list next to the View button (circled above) to switch to SQL view.  This is the SQL statement, separated into logical sections for ease of interpretation:



Orders.OrderID, Orders.CustomerID, Orders.EmployeeID, Orders.OrderDate, Orders.RequiredDate, Orders.ShippedDate, Orders.ShipVia, Orders.Freight, Orders.ShipName, Orders.ShipAddress, Orders.ShipCity, Orders.ShipRegion, Orders.ShipPostalCode, Orders.ShipCountry,


Customers.CompanyName, Customers.Address, Customers.City, Customers.Region, Customers.PostalCode, Customers.Country


FROM Customers


     ON Customers.CustomerID = Orders.CustomerID;

Note:  The table names need not be repeated unless the same column names exist in both tables.  The table names are only required in the FROM, JOIN, and ON clauses, and in the latter, only because the relating column, CustomerID, has the same name in both tables.  

The query syntax shown above follows ANSI (American National Standards Institute) rules and should work in the latest versions of all relational databases.  Older syntax includes the join condition in the WHERE clause (theta style).  Note the number of rows and columns in the result set for the Orders Query and try the same example (with fewer columns), using the older style and table aliases, as follows:


SELECT o.OrderID, o.EmployeeID, o.OrderDate, o.RequiredDate, o.ShippedDate, o.ShipVia, o.Freight, c.CompanyName, c.Address, c.City, c.Region, c.PostalCode, c.Country

FROM Customers c, Orders o

WHERE c.CustomerID = o.CustomerID;

Note for MS Access users:  Compare this query in design view with the ANSI style query.  MS Access runs the query correctly but cannot represent it in the usual way In the graphical query interface.



The JOIN operator specifies how to relate tables in the query.  The JOIN operator is one of the set operations available in relational databases.


The following join types of join are available in most relational databases:



            OUTER (LEFT. RIGHT, FULL)


Joins may be represented as Venn diagrams, as shown below along with other common set operations:

join types

Result of applying these joins in a query:

INNER JOIN:  Select only those rows that have values in common in the columns specified in the ON clause.


LEFT, RIGHT, or FULL OUTER JOIN:  Select all rows from the table on the left (or right, or both) regardless of whether the other table has values in common and (usually) enter NULL where data is missing.  (Note:  FULL OUTER JOIN not implemented in Access.)


CROSS JOIN (not illustrated - not exactly a set operation):  Select all possible combinations of  rows and columns from both tables (Cartesian product). Not available in Access but can "happen" by not specifying relationships between tables or not setting up the appropriate joins in a query.  (Not A Good Thing - the query may run for a very long time  and produce a huge, not very useful result set.)


Access uses the ANSI  (American National Standards Institute) style, with the JOIN and ON keywords.  Access, MySQL, and Oracle all use similar syntax, with more join types and options and other set operations in MySQL and Oracle (CROSS JOIN, FULL OUTER JOIN, INTERSECT, MINUS).


Select from two tables:  More examples


SELECT Products.*, Categories.CategoryName

FROM Categories
    INNER JOIN Products
    ON Categories.CategoryID=Products.CategoryID

WHERE (((Products.Discontinued)=No));



 [Order Details].OrderID, [Order Details].ProductID, Products.ProductName, [Order Details].UnitPrice, [Order Details].Quantity, [Order Details].Discount,
CCur([Order Details].UnitPrice*[Quantity]*(1-[Discount])/100)*100 AS ExtendedPrice

FROM Products
    INNER JOIN [Order Details]
    ON Products.ProductID=[Order Details].ProductID

ORDER BY [Order Details].OrderID;

       Note:  The calculation is not as complex as it may seem.  It is simply unit price * quantity * discount, formatted as currency.

Select from two tables:  Exercises




            City, Country      (e.g.,  Montreal, Canada) 


         with a column header such as ShippedTo or Shipped To.  To do this, replace the City and Country columns with one calculated column (comma at the end to separate from the next column if necessary):


            City & ", " & Country AS ShippedTo,


           or      City & ", " & Country AS [Shipped To],

Note:  Be sure to find the correct names for the City and Country columns - they are different in the two tables.  (Result: 58 rows)


            City,  Region PostalCode                         (e.g.:  Newark, DE 19716)


         Can you sort on the calculated column in the SQL statement?  (Result:  152 rows)




(Recommended, ANSI-style)


            SELECT column list

            FROM table1

                     &nb sp;  INNER JOIN table2

                     &nb sp;  ON table1.col1=table2.col2

            WHERE criteria

            ORDER BY column list


(Older, theta-style)


            SELECT column list

            FROM table1, table2

                    WHERE table1.col1=table2.col2

                    AND other criteria

            ORDER BY column list


- col1 in table1 is usually that table's primary key

- col2 in table2 is a foreign key in that table

- col1 and col2 must have the same data type and for certain data types, the same size







   Run the query - notice that there is one row per product.   Then switch to SQL view:

SELECT Categories.CategoryID, Categories.CategoryName, Products.ProductName, Sum([Order Details Extended].ExtendedPrice) AS ProductSales

FROM Categories
     INNER JOIN (Products
          INNER JOIN (Orders
               INNER JOIN [Order Details Extended]
               ON Orders.OrderID=[Order Details Extended].OrderID)
          ON Products.ProductID=[Order Details Extended].ProductID)
     ON Categories.CategoryID=Products.CategoryID

WHERE (((Orders.OrderDate) Between #1/1/1997# And #12/31/1997#))

GROUP BY Categories.CategoryID, Categories.CategoryName, Products.ProductName

ORDER BY Categories.CategoryName;

The number of joins is equal to the total number of tables (or views) minus one.
A join condition (ON table1.col1 = table2.col2) must be specified for each join.

If the join is in the WHERE clause, the rules are the same - the minimum number of join criteria is equal to the number of tables (or views) minus one.

The GROUP BY clause summarizes data in subsets, in this case giving one row per product.  (Topic to be covered in detail in the third class)

The order of clauses in the SQL statement is important:  GROUP BY after WHERE (if present), ORDER BY last.


 Note:  Relationships among the six tables are not linear so it is harder to "see" them in the SQL statement.




Tip:  To analyze or troubleshoot a query in the Access query window or in the command line utility in Oracle or MySQL, try breaking the statement as shown in the syntax diagram, with the keywords at the beginning of the lines; or copy and paste to a text editor (e.g., Notepad) and rearrange there.





Used to find data in one table that is missing related data from another, for example a supplier from whom we have no products, or a product that hasn't been categorized, or a customer who has not placed an order.  


Principle:  Join the tables and find all the rows from one table whose corresponding rows in the other table have a null value (data missing or value unknown).





List the company name, contact person, and phone number of customers who have not placed orders.  Type the following statement in the SQL window:


SELECT CompanyName, ContactName, Phone

FROM Customers

     LEFT JOIN Orders

     ON Customers.CustomerID = Orders.CustomerID

WHERE Orders.CustomerID is null;

The result should be 2 rows.


In MS Access, this statement is similar to the SQL generated by the Unmatched Query Wizard.

Note the emphasis on "related data" above.   See whether an outer join is necessary to create a list like the one in the previous example of customers whose orders have not been shipped.  Why or why not?



Queries using set operations




A UNION query brings together in one result set data from two or more unrelated tables or queries that have identical structure (same number of columns with same data types occurring in the same order; not necessarily same column headers).


A UNION query cannot be built in the graphical query interface in Access.




Customers and Suppliers by City:

SELECT City, CompanyName, ContactName, "Customers" AS [Relationship]
FROM Customers
UNION SELECT City, CompanyName, ContactName, "Suppliers"
FROM Suppliers
ORDER BY City, CompanyName;


- Variable number of SELECT statements linked by the key word UNION

- Columns must be named (important if they were calculated)

- Optional additional column or columns to add information or to make table structures match

- No duplicates unless UNION ALL is specified (not obvious from this example)

- If the result set is to be sorted, only one ORDER BY clause at the end


UNION:  Exercises









            SELECT statement1


            SELECT statement2



            SELECT statement-last

            ORDER BY column list



            SELECT statement1

            UNION ALL


            SELECT statement-last

            ORDER BY column list



Other set operators (Oracle):  INTERSECT and MINUS




Used when linking one database to another through an ODBC (Open Database Connectivity) or JDBC (Java Database Connectivity  or Sun Java standard) connection.  The query written in the local database is "passed through" as is to the database on the server and processed by the remote database.




- Update values in a remote table

- Find the next sequence number for a table (Oracle - uses the auxiliary table DUAL in the remote database)

- Run a query using a function or calculation that cannot be performed in the local database