4_Structured_queries_method_l2_v1
Оценка 4.9

4_Structured_queries_method_l2_v1

Оценка 4.9
docx
09.05.2020
4_Structured_queries_method_l2_v1
4_Structured_queries_method_l2_v1.docx

So, where is the SQL?

The point of the Query-By-Example editor is to make it simple to create queries in MS Access. What actually happens is that MS Access converts the Query-By-Exampleobjects into SQL. Wouldn’t it be great if you could see the generated SQL? You bet it would!

To view the SQL code:

  • Select the down arrow of the view button in the top left hand corner of the screen.

Select SQL view.

https://www.accessallinone.com/wp-content/uploads/2018/10/image004a.png

Figure 4

The generated SQL code is shown and reads:

https://www.accessallinone.com/wp-content/uploads/2018/10/image005a.png

Figure 5

The Query-By-Example editor can be used instead of writing pure SQL which is both more convenient and a useful learning tool.

Summarising the SQL

Let’s summarise some of the details of the SQL statement in Figure 5.

  • The SELECT keyword comes first. Then the table fields are referred to explicitly. This means that the fields are prefixed with the table name, and separated by a full stop (tblStudents.StudentID). 
  • Each of the table fields in the query is separated by a comma.
  • The FROM keyword tells the database which table the table fields refer to.
  • The end of the SQL command is indicated by the semi-colon (this semi-colon is not necessary when working with single queries).

Altering the SQL directly

In the above example, we began with the query in design mode. We then constructed a query using Query-By-Example before viewing the query as pure SQL. Let’s see what happens if we modify a pure SQL query and then view it using the Query-By-Example editor.

Using the pure SQL from Figure 5:

Place the cursor in the Query text between the tblStudents.EmailAddress field and the FROM keyword. Delete the tblStudents.EmailAddress field and the preceding comma. Your query should now look like this:

https://www.accessallinone.com/wp-content/uploads/2018/10/image006a.png

Figure 6

 

If you now switch to design view (View > Design View) you will see that the tblStudents.EmailAddress field has been removed from the Query Grid.

https://www.accessallinone.com/wp-content/uploads/2018/10/image007b.png

Figure 7

So, you have two options for creating SQL: through the Query-By-Example editor or using pure SQL.

Selecting Records by Criteria

The SELECT query is useful as it provides a simple way to extract data from the database. But we do not always want to retrieve every row from a table. Sometimes, we want to retrieve a subset of rows that match a certain criteria.

For example, we may only want to see records from the tblStudents table where the city is ‘New York’. Or we may only want to return employees from the tblEmployees table that have the job title of ‘Secretary’. These actions are achieved by using the WHERE clause.

  • Open the QBE editor, remove any other tables and add the tblStudents table.
  • Add StudentID, FirstNameLastName and City to the Query Grid
  • In the City column add the text “New York” to the Criteria row and press Enter

Your Query Grid should look like Figure 8:

 

https://www.accessallinone.com/wp-content/uploads/2018/10/image008a.png

Figure 8

  • Run the query and you will see that the only rows returned are those that have ‘New York’ in the city column.

 

 

 

 

https://www.accessallinone.com/wp-content/uploads/2018/10/image009a.png

Figure 9

  • Now switch to SQL view to see the SQL code for this query:

 

https://www.accessallinone.com/wp-content/uploads/2018/10/image010a.png

 

Figure 10

You can see that we have the standard SELECT query but a WHERE clause has been added to filter the records based on the criteria shown.

The WHERE clause comprises of the WHERE keyword followed by the field name tblStudents.City the comparison operator ‘=’ and the literal value that is being searched for “New York”.

You will notice that in converting it from a QBE query to pure SQL, parentheses have been added around the WHERE claus. This particular feature is a small annoyance that MS Access has, in that the parentheses are not necessary (certainly not in a simple query like this). The SQL could just as well read:

SELECT tblStudents.StudentID, tblStudents.LastName, tblStudents.FirstName, tblStudents.City FROM tblStudents WHERE tblStudents.City=”New York”;

The use of the WHERE clause is a very important concept to understand, particularly when using action queries such as UPDATE and DELETE (more on this later).

Comparison Operators

In the WHERE clause we have used a comparison operator (the equals sign). It is often useful to use other comparison operators such as:

  • < Less than,
  • > Greater than,
  • <> Not equal to,
  • <= Less than or equal to,
  • >= Greater than or equal to.

In the following example, we are using the ‘>’ operator to find all courses that cost more than £119:

 

https://www.accessallinone.com/wp-content/uploads/2018/10/image011a.png

 

Figure 11

Comparison operators work intuitively with numeric values. When comparing strings, the greater than operator (>) will look for a string that comes after another string alphabetically. So, it is correct to say Dog > Cat (D comes after C) but incorrect to say Duck > Monkey(M does not come after D).


 

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So, where is the SQL? The point of the

So, where is the SQL? The point of the

So, you have two options for creating

So, you have two options for creating

Figure 9 Now switch to

Figure 9 Now switch to

Less than or equal to, >=

Less than or equal to, >=
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