sql unit 6 views and temporary tables

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SQL Unit 6Views and Temporary Tables

Kirk Scott

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• 6.1 Introduction• 6.2 Enabling Database Content in MS Access• 6.3 Temporary Tables• 6.4 Views• 6.5 Inline Views• 6.6 Details

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6.1 Introduction

• It is possible to write relatively complex SQL queries.

• Queries can include more than one table, have multiple conditions, etc.

• When queries become complex, people don't just sit down and write them from beginning to end.

• They break the problem into pieces, try to solve the pieces, and then put the solutions together into a whole.

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• In previous units some examples have come up where something can't be done in Access SQL.

• For example, this query won't work:• • SELECT COUNT(DISTINCT stickerprice)• FROM Car

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• It is perfectly reasonable to want to count the number of distinct values of the stickerprice field in the Car table, but this syntax isn't supported.

• This obviously has two parts, count, and distinct, and it is reasonable to think that you could get the solution by doing it in steps.

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• This would be a plan of action:• 1. Create a new table which would just hold

stickerprice values.• 2. Write a query to find the distinct

stickerprices and enter them into the table.• 3. Write a query that would count the

number of stickerprices in the new table.

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• Step two, entering the results of the first query into a table, is obviously impractical as given.

• If you're going to enter the results of the query into the new table by hand, you might as well just count them by hand.

• However, as you will see, there is SQL syntax for creating a table in mid-stream, using the results of a query

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6.2 Enabling Database Content in MS Access

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• Using SQL it will be possible to create a new table out of the results of a query

• If you try to run a query in MS Access that creates a table you may not have success.

• The query will obviously not run, and the only indication of what's wrong will consist of a temporary message that appears at the bottom of the Access window:

• "The action or event has been blocked by Disabled Mode."

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• The overheads in this section explain how to deal with this situation

• You will not be tested on this stuff• It is just presented in case you run into

problems when doing homework• If you forget the steps shown below, all you

have to do is go to Help in the menu and search for "Disabled Mode" for an explanation.

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• The screen shot on the next overhead shows the “blocked by Disabled Mode” message as it appears at the bottom of the Access screen

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• It is possible that in your installation of MS Access the so-called Message Bar already appears

• If so, you can just breeze through the following steps• If the Message Bar doesn’t appear, then the first step in

dealing with the “blocked by Disabled Mode” problem is to make sure that the Message Bar appears on the screen.

• To show the Message Bar, go to the Database Tools tab and check the Message Bar box.

• See the screen shot below and find the Tools/Message Bar check box.

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• The screen shot below shows the window with the Message bar showing

• It has a Security Warning which you would see after you’d tried to run a query in Disabled Mode.

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• The Message bar includes an Options button.• Once you’ve got the Options button showing,

the next step is to click on it.• When you do so, you’ll get a Security Alert box

with an option allowing you to “Enable this content”

• You should select that option.

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• After selecting the option, you should click OK.• After taking all of these steps, it should be

possible to run the query—but not necessarily immediately.

• After clicking OK in the Security Alert, it may be necessary to close the query and then try to run it again before it will work.

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6.3 Temporary Tables

• There is SQL syntax which makes it possible to create a table directly from query results.

• Such a table is known as a temporary table.• The table is not really temporary, but it is

different from the tables that exist in the original database design

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• Because of the interruption of describing how to deal with disabled content, you may have forgotten why it is convenient to be able to create a table from query results.

• The goal is to divide and conquer, obtaining a complex query result by working with two parts.

• The first part consists of making the temporary table.• The second part consists of writing a query that uses

the temporary table.

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• This is the syntax that doesn’t work in Access:

• SELECT COUNT(DISTINCT stickerprice)• FROM Car

• The goal is to get the desired results by writing two queries that are related by the fact that the first creates a temporary table and the second uses it.

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• 1. The following query accomplishes the first part.

• It selects the distinct stickerprices from the Car table, creating the temporary table Distinctpricetable in the process.

• SELECT DISTINCT stickerprice INTO Distinctpricetable

• FROM Car

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• 2. The following query accomplishes the second part.

• It selects the count of the stickerprices in the temporary table, Distinctpricetable.

• • SELECT COUNT(stickerprice)• FROM Distinctpricetable

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• It is important to understand that even though the intermediate table created by this process, Distinctpricetable, is known as a temporary table, it is persistent.

• It will show up in Access under the table tab as a real live table containing data.

• It will contain a snapshot based on the contents of the Car table at the time the query was run that created it.

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• Once Distinctpricetable has been created, subsequent changes to the Car table will have no effect its contents

• On the other hand, if you run the query that creates Distinctpricetable again, you will wipe out whatever it previously contained

• If changes to the Car table have been made in the meantime, then the new version of Distinctpricetable will reflect them

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6.4 Views• 1. The topic of views will be introduced by solving the problem

of the previous example in yet another way. • The following query generates results containing the distinct

stickerprices of the Car table• Notice that the results of the query are not saved as a temporary

table.• • SELECT DISTINCT stickerprice• FROM Car

• Let the query itself be saved under the name Distinctpricequery.

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• 2. The saved query can play the role of a table in another query.

• This query counts the number of stickerprices in the results of the saved query.

• • SELECT COUNT(stickerprice)• FROM Distinctpricequery

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• Distinctpricequery is playing the same role here as Distinctpricetable did in the previous approach.

• It appears after FROM, and is the thing that this query selects results from

• It is important to understand that although the results are the same, the two approaches work differently.

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• 3. Views will now be explained in detail. • In SQL the term "view" is used to describe

defining a query and saving that query for future use.

• The SQL syntax, not surprisingly, starts with "CREATE VIEW".

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• Because Microsoft Access supports the naming and saving of queries directly, there is no need to use the SQL syntax.

• A view is by definition a saved query. • The idea will be illustrated with more

examples.

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• The query below selects the vin and all of the monetary fields from the Car table.

• Let the query be saved under the name Carmoneyview.

• • SELECT vin, stickerprice, dealercost• FROM Car

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• 4. It is the SQL code that is saved under the name Carmoneyview, not the results of the query.

• This saved query can be used just like a table in another query.

• • SELECT vin, stickerprice• FROM Carmoneyview

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• 5. To emphasize the point that what you’re using is a saved query (not a temporary table), suppose that the query above was run.

• Then the following new record is entered into the base table, Car.

• • 'qqqqq', 'Chevrolet', 'Impala', 2002, 8000.00,

7500

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• 6. Then suppose the same query which selects from the saved query is run again.


• The results of this query on Carmoneyview will be different from what they were before the insertion of the new record into Car.

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• The results of the query will include a record with the values 'qqqqq' and 8000.00.

• The results change because every time this query is run, the underlying saved query, Carmoneyview, is run again

• The results of the Carmoneyview query will always reflect the current state of the Car table, including any changes that have been made in it since the last time the query was run.

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• 7. Now consider changing the saved query, Carmoneyview, so that its results would be ordered.

• • SELECT vin, stickerprice, dealercost• FROM Car• ORDER BY stickerprice

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• 8. Then if you ran this query again, which uses the saved query like a table, its results would also be ordered.


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• 9. It may be convenient to rename the fields when defining a view.

• Let Carmoneyview be redefined in this way using column aliases.

• • SELECT vin AS v, stickerprice AS sp, dealercost

AS dc• FROM Car

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• 10. Then if you used the view in another query, you would have to use the new field names.

• • SELECT v, sp• FROM Carmoneyview

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• 11. It is important to understand that you can use a view in a query in any way that a table can be used.

• Here is a join query that uses the view Carmoneyview.• • SELECT v, sp, salesprice• FROM Carmoneyview, Carsale• WHERE v = vin• • Notice that the names of the fields in the joining condition

don't have to be qualified with table names because vin was given the alias v in Carmoneyview.

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• 12. As noted earlier, if you change the base table, the next time you make use of a saved query on that table, any changes in the base table will be reflected.

• This process also goes in the other direction. • If you insert, update, or delete records in the

view, these changes go directly to the base table.

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• The base table name and the view name are equally valid handles for doing anything that it's possible to do in SQL, including changing the contents of a table.

• The SQL syntax for inserting, updating, and deleting will be covered in full in Unit 10.

• However, a small example will be given here that uses that syntax and illustrates the power of a view as a handle on a table

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• This example has the effect of finding a specified record using the view and then changing one of the field values for it:

• • UPDATE Carmoneyview• SET sp = 5000• WHERE v = 'qqqqq'

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• 13. If you then ran the following query on the Car table, you would discover that the stickerprice for the car with vin = 'qqqqq' is now 5000.

• • SELECT vin, stickerprice• FROM Car• WHERE vin = 'qqqqq'

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• A view is always based on one or more real tables.

• If you make changes to the fields selected in the view, the changes are actually made in the table that the view is based on.

• The name of the view is just as valid a handle for referring to the base table as the name of the table itself is.

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• A student asked once whether this functionality of a view depends on its containing the primary key value

• In theory it doesn’t, and in practice, in Access, it doesn’t

• An example emphasizing this idea follows

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• The following could be saved as MyView:

• SELECT stickerprice AS sp, dealercost AS dc• FROM Car

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• This query updates all of the relevant rows of the Car table even though the view doesn’t contain the primary key of the Car table:

• UPDATE MyView• SET sp = 5000• WHERE dc = 16500

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• 14. A final way of describing what is happening with saved queries is that any results generated by a saved query are completely transient.

• The query is saved, but the results are not saved. • Every time a query is run which uses a saved

query in the role of a table, the results of the saved subquery are generated again at run time based on the values in the table or tables contained in the saved query’s definition.

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• Temporary tables and views can both be used to divide and conquer a problem

• In a sense, their uses in this regard are similar• However, there is a big difference between

temporary tables and views.

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• Temporary tables are misnamed.• They aren’t temporary, they’re persistent.• They are also static.• Once they’ve been created, changes to the

tables that they were based on don’t affect the contents of the temporary table.

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• Views are saved queries.• What is saved is the SQL syntax, not the

results of running it.• The saved syntax is persistent, but the results

are not.

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• Every time the view is used, its results are generated or materialized again.

• This means that views are dynamic. • Changes to the tables that they were based on

are reflected in the results every time a view is used.

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6.5 Inline Views

• 1. The term "inline view" refers to the idea that any query that makes use of a view could be rewritten with the view simply appearing as a parenthesized, nested subquery in the overall query.

• As above, let this query be saved as Carmoneyview:• • SELECT vin, stickerprice, dealercost• FROM Car

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• Then, as shown, it's possible to write this query:


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• The query shown below is literally what is executed at run time.

• It is also a perfectly legitimate query in its own right.

• • SELECT vin, stickerprice• FROM• (SELECT vin, stickerprice, dealercost• FROM Car)

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• At run time, the inner query inside the parentheses is run first, bringing its results into existence in real time, and then the outer query operates on those results to produce the final results.

• The inner, parenthesized part of the query is a subquery

• This subquery is an inline view.

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• So when solving a problem by dividing and conquering, the first part of the solution doesn’t have to be saved as a separate query or view

• All you have to do is remember it, and write the second part of the solution using it

• The example query isn’t particularly useful, but it could literally be written and run as shown.

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• The inline view has another important characteristic that stems from the way views work.

• The inline view, the complete, parenthesized subquery, is playing the role of a table in the FROM clause of the outer query.

• Even though the inline view is contained within parentheses, its fields are available for use in the outer query.

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• 2. The problem of counting the distinct salesprices gives a more useful example.

• As shown earlier, let this query be saved as Distinctpricequery:

• • SELECT DISTINCT stickerprice• FROM Car

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• Then it's possible to write this query:• • SELECT COUNT(stickerprice)• FROM Distinctpricequery

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• This is literally what is executed at run time:• • SELECT COUNT(stickerprice)• FROM• (SELECT DISTINCT stickerprice• FROM Car)• • The outer query is free to make use of the field

stickerprice, which is defined in the Car table and selected in the results of the inline view

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• 3. Nested queries in general are an important topic.

• Here are a few more examples. • You would probably not write these queries

using inline views, but they do illustrate in greater detail how inline views work.

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• Here is a query that accomplishes the same thing that could be done with a query using the keyword AND:

• • SELECT vin, stickerprice• FROM• (SELECT vin, stickerprice, dealercost• FROM Car• WHERE dealercost < 10000)• WHERE stickerprice > 10000

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• Here is a query that accomplishes the same thing that could be done with a non-nested join:

• • SELECT vin, stickerprice, salesprice• FROM Carsale,• (SELECT vin AS v, stickerprice, dealercost• FROM Car)• WHERE vin = v

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• In this query, the joining fields vin and v "cross" parenthesis boundaries.

• Fields defined in the inner query can be used in the outer query.

• It's convenient to give vin the alias v in the nested query.

• This makes it possible to SELECT vin and write the joining condition in the WHERE clause as vin = v without having to qualify vin with the respective tables it appears in.

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• It would also be possible to write the query this way:

• • SELECT Car.vin, stickerprice, salesprice• FROM Carsale,• (SELECT vin, stickerprice, dealercost• FROM Car)• WHERE Carsale.vin = Car.vin

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6.6 Details

• Inline views are one of those cases where the Microsoft Access design view can become inconvenient.

• In all of the examples in this set of notes, the simplest possible syntax was shown.

• Unfortunately, if you save the query and then go back later to edit it, it will frequently look different from the way you typed it in.

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• For example, table or field aliases may be set off with square brackets or quotes, dot notation may be used in unexpected places, and so on.

• The original inline view query is shown below, followed by what you might see if you saved it and opened it again in the SQL view.

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• SELECT vin, stickerprice• FROM• (SELECT vin, stickerprice, dealercost• FROM Car)

• SELECT vin, stickerprice• FROM (SELECT vin, stickerprice, dealercost• FROM Car) AS [%$##@_Alias];

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• It doesn't make sense to try to master this complex (and needless) notation, but you shouldn't be surprised when you run into it.

• Unfortunately, it’s potentially unavoidable.

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• You may write an inline query and find that it doesn't produce the desired results.

• If you saved it and reopen it to fix it, you will have a choice:

• Try to deal with Access’s crazy changes to what you wrote, or try to write the correct query again from scratch.

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The End

sql unit 6 views and temporary tables

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