Databases Tutorial 2 Further Select Statements

Objectives for Week Data types Sort retrieved data Formatting output

Some additions to tutorial 1 You do not need to use quotes for numeric fields: –Select * from Product where UnitPrice <5 Note the format of the SELECT statement SELECT [{columnname}]/* FROM table-name [WHERE condition] Both SELECT and FROM are always needed.

Sorting The order by clause will allow you to sort the fields. The default order is ascending. To change it to descending, use the keyword desc. select * from categories order by categoryName is the same as select * from categories order by categoryName asc To use descending order: select * from categories order by categoryName desc

To put a name on a column Transact SQL allows the use of the ‘as’ clause to give a name to a column. –Unitprice as Price or –UnitPrice as ‘Unit Price’ This can be used on any column, but is especially useful in a derived column. New columns can be derived from existing fields: E.g. the value of an item in stock is the number in stock by the unit price.

Cast and CONVERT CAST and CONVERT Explicitly converts an expression of one data type to another. CAST and CONVERT provide similar functionality. This can be used to format output and convert data fields from one format to another

Cast and Convert CAST and CONVERT –Explicitly converts an expression of one data type to another. CAST and CONVERT provide similar functionality. Syntax –Using CAST: –CAST ( expression AS data_type ) –Using CONVERT: –CONVERT ( data_type [ ( length ) ], expression [, style ] )

Number conversion When data types are converted with a different number of decimal places, the value is truncated to the most precise digit. –For example, the result of SELECT CAST( AS int) is 10. When data types in which the target data type has fewer decimal points than the source data type are converted, the value is rounded. – For example, the result of CAST( AS money) is $

B. Use CAST with arithmetic operators This example calculates a single column computation (Copies) by dividing the total year- to-date sales (ytd_sales) by the individual book price (price). This result is converted to an int data type after being rounded to the nearest whole number. –USE pubs –GO –SELECT CAST(ROUND(ytd_sales/price, 0) AS int) AS 'Copies' FROM titles GO

C. Use CAST to concatenate This example concatenates noncharacter, nonbinary expressions using the CAST data type conversion function. –USE pubs –GO –SELECT 'The price is ' + CAST(price AS varchar(12)) FROM titles WHERE price > –GO

D. Use CAST for more readable text This example uses CAST in the select list to convert the title column to a char(50) column so the results are more readable. –USE pubs –GO –SELECT CAST(title AS char(50)), ytd_sales FROM titles WHERE type = 'trad_cook' –GO

E. Use CAST with LIKE clause This example converts an int column (the ytd_sales column) to a char(20) column so that it can be used with the LIKE clause. –USE pubs –GO –SELECT title, ytd_sales FROM titles WHERE CAST(ytd_sales AS char(20)) LIKE '15%' AND type = 'trad_cook' –GO

Syntax of CAST Using CAST: CAST ( expression AS data_type ) This allows us to do the following: –Format the output so that it is more readable. –Change a number to a character string. –Change a character string to a numeric format.

Using CAST

Exercises Retrieve the system date, using the ‘GetDate’ function. Convert it into char(12) format. Display it using style 3. Now display it using styles 1 through 14, noting the differences. Which of these formats do you think would be most useful in this country and for what purposes?

Exercises (Northwind) To demonstrate the effectiveness of ‘order by’: –Select all from the order details table –Select all from the order details table ordered by product id. –Select all from the order details table in descending product id order. –Show the order id, the customer id and the day, month and year of the order date, for all orders, in customer id order.

Exercises Amend the previous exercises to put names on the columns. Format your outputs so that they are easy and pleasant to read.

Achievements Writing Basic SQL Select Statements –[√]List the capabilities of SQL SELECT statements –[ √ ]Execute a basic SELECT statement Restricting and Sorting Data –[ √] Limit the rows retrieved by a query –[ √] Sort the rows retrieved by a query Single-Row Functions –[ √] Use conversion functions