Query Lab

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Log on to PMA (PHPMyAdmin) and click on the Northwind database

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Alternative place to work: http://www.w3schools.com/sql/trysql.asp?filename=trysql_select_all

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After choosing the Northwind database, click on the Query tab

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Use the drop-down to choose `customers`.* Then check the Show checkbox. Then click The Update Query button

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The marks around the table name seen here are called tick marks or back-ticks. They slant and are above the Tab on the upper left of the keyboard. They are NOT single quotes. They are not necessary in this case but PMA includes them.

Key words

• SELECT– In SQL a SELECT statement is one that looks at a

subset of data– As opposed to adding data (INSERT), taking away

data (DELETE) and changing data (UPDATE)

• FROM– FROM indicates the source of the data – which

tables that data comes from

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Next click the Submit Query button

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Result: 94 records (displaying 30 per page)

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Click on Northwind to move from the specific customers table to the more general Northwind database

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Click on the Query tab. This time select specific fields (instead of * for all fields). Remember to check Show and click Update Query.

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Result from clicking the Submit Query button. It produces the same number of records but fewer fields per record.

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Projection

• In “relational algebra” the previous query is an example of a projection – we have projected out some of the fields.

• (Don’t be intimidated by the term “relational algebra” it just means things we can do to database tables.)

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YOU DO #1

• Make a Word document and paste into the SQL for – The first name and last name of Employees

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Return to Northwind/Query. Choose `customers`.* and Show for one column and `customers`.`City` and ='London' as the Criteria

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Click Update to show the query

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WHERE – selection

• The new keyword appearing is WHERE – It is used to limit the record that will display to

those that meet some condition

• In relational algebra, this procedure of limiting the number of records is known as selection

• Note in this case that the WHERE condition uses single quotes – different from ticks

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Result of selecting customers whose city is London. 4 records.

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Compound condition: City is London or Berlin

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Results of Compound condition: City is London or Berlin

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YOU DO #2

• Paste into your Word document the SQL for – Suppliers whose Country is France or Germany

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Compound condition: contactTitle is Owner and Country is Mexico

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Result of Compound condition: contactTitle is Owner and Country is Mexico

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YOU DO #3

• Paste into your Word document the SQL for– Employees whose Title is sales Representative and

Country is USA

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Combining selection and projection

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selectionprojection

Result of selection and projection

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Sort result by choosing Ascending or descending from the drop-down under a field.

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Keywords for sorting

• ORDER BY– Is followed by the field (or fields) upon which the

sorting will be based• ASC– Indicates the results will be shown from lowest to

highest numerically or in alphabetic order for text/strings

• DESC– Highest to lowest or reverse alphabetical

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Results of query sorted on CompanyName

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YOU DO #4

• Paste into your Word document the SQL for – Products sorted by ProductName

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Obtaining a list of customer countries

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Result of customer country query (so far). Click on Edit to work on further.

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SQL editing dialog box arises

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Add keyword DISTINCT to eliminate duplicate country results

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Click Go. Query updated. And only unique countries occur in result

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YOU DO #5

• Paste into your Word document the SQL for– Distinct cities of employees

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Criteria other than equality: Products with a UnitPrice less than 20

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Result of Products with a UnitPrice less than 20

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Products with a UnitPrice greater than or equal to 30

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Result of Products with a UnitPrice greater than or equal to 30

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Switching >= to >

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A BETWEEN query criteria: UnitPrice between 20 and 30

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Results of BETWEEN query: UnitPrice between 20 and 30

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YOU DO #6

• Paste into your Word document the SQL for– Products having a UnitsInStock less than or equal

to 15

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