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Mathematical Functions
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Mathematical functions manipulate numeric values according to mathematical rules. The ROUND function is presented in detail, including an example, followed by a table that lists other mathematical functions that are commonly found in
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SQL Demysti ed
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SQL implementations. As always, check your DBMS documentation for a speci c list of supported mathematical functions.
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The ROUND function rounds off a number to the speci ed number of decimal places. The number is provided as the rst expression and the number of decimal places as the second. The general format of the ROUND function is shown next, followed by an example adapted from earlier in the chapter.
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ROUND (numeric_expression,number_of_decimal_places)
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What is the average cost of a copy of The Last Samurai, rounded to the nearest penny
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SELECT ROUND((RETAIL_PRICE_VHS + RETAIL_PRICE_DVD) / 2, 2) AS AVG_COST FROM MOVIE WHERE MOVIE_TITLE = 'The Last Samurai'; AVG_COST ---------17.96
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Other Mathematical Functions
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The table that follows lists the most commonly found mathematical functions. The general syntax for all of them is the same:
FUNCTION_NAME(expression)
Function ABS COS EXP POWER SIN TAN Description Absolute value of the given number Trigonometric cosine of the given angle (in radians) Exponential value of the given number Raise number to a power (both number and power are parameters) Trigonometric sine of the given angle (in radians) Trigonometric tangent of the given angle (in radians)
Conversion Functions
Conversion functions convert data from one data type to another.
Retrieving Data Using Data Query Language
CAST
The CAST function converts data from one data type to another. Note, however, that it is not yet supported by DB2. Here is the general syntax of the CAST function, followed by an example:
CAST (expression AS data_type)
List the DVD price of The Last Samurai with a leading dollar sign. The numeric value must be converted to (cast as) a character string so that you can concatenate it with a literal containing a dollar sign. Note that for Microsoft SQL Server, the concatenation operator in the statement has to be changed from || to + .
SELECT '$' || CAST(RETAIL_PRICE_DVD AS VARCHAR(6)) AS PRICE FROM MOVIE WHERE MOVIE_TITLE = 'The Last Samurai'; PRICE ------$19.96
CONVERT TO
Many DBMS implementations offer a CONVERT or CONVERT TO function. However, the CAST function is generally recommended instead because it is implemented in a more standard way across vendors.
Aggregate Functions and Grouping Rows
An aggregate function is a function that combines multiple rows of data together into a single row. The following table shows aggregate functions that are supported in most SQL implementations:
Function Name AVG COUNT Description Calculates the average value for a column or expression. Counts the number of values found in a column. The DISTINCT keyword can be used to count the number of unique values instead of the total number of values (rows) in a column. Finds the maximum value in a column. Finds the minimum value in a column. Sums (totals up) the values in a column.
MAX MIN SUM
Here are some examples:
SQL Demysti ed
What is the average price of a DVD Notice that the ROUND and AVG functions are nested so you get the result in dollars and cents.
SELECT ROUND(AVG(RETAIL_PRICE_DVD),2) AS AVG_PRICE FROM MOVIE; AVG_PRICE ---------24.67
How many movies are in the MOVIE table
SELECT COUNT(*) AS NUM_MOVIES FROM MOVIE; NUM_MOVIES ---------20
How many different movie genres are represented in the MOVIE table Notice the use of DISTINCT so the DBMS counts unique values of MOVIE_GENRE_CODE.
SELECT COUNT(DISTINCT(MOVIE_GENRE_CODE)) AS NUM_GENRES FROM MOVIE NUM_GENRES ---------5
What is the shortest and longest movie title Notice the nesting of the LENGTH function with the MIN and MAX functions. This version works in Oracle, DB2, and MySQL (change LENGTH to LEN for Microsoft SQL Server and Sybase).
SELECT MIN(LENGTH(MOVIE_TITLE)) AS MIN_LENGTH, MAX(LENGTH(MOVIE_TITLE)) AS MAX_LENGTH FROM MOVIE; MIN_LENGTH MAX_LENGTH ---------- ---------7 54
Retrieving Data Using Data Query Language
GROUP BY Clause
As you have seen, if you use an aggregate function by itself in a query, you get one row back for the entire query. This makes sense because there is no way for the RDBMS to know what other result you might want, unless you tell it, which is the very purpose of the GROUP BY clause. It tells the DBMS to form the rows selected by the query into groups based on the values of one or more columns and to apply the aggregate function(s) to each group, returning one row for each group in the result set. This is much like asking for subtotals by department instead of one grand total for an entire company, but as you have seen, aggregate functions can do much more than just add things up. By the way, the DBMS will sort the rows selected by the query on the columns listed in the GROUP BY clause (so it can group them easily), so the groups will be returned in ascending sequence unless you add an ORDER BY that speci es another sequence. Here is an example: List each movie genre code with the number of movies that are assigned to it.
SELECT MOVIE_GENRE_CODE AS GENRE, COUNT(*) AS COUNT FROM MOVIE GROUP BY MOVIE_GENRE_CODE; GENRE COUNT ----- ---------ActAd 7 Comdy 4 Drama 6 Forgn 1 Rmce 2
What happens if you leave the GROUP BY clause out of this query The DBMS returns an error message, and unfortunately, the error message is often quite cryptic. Most newcomers to SQL have dif culty understanding the problem, but without the GROUP BY clause, the query is illogical. The COUNT(*) function is an aggregate function, so without a GROUP BY, it returns a single row of data. However, the MOVIE_GENRE_CODE is a column and without being named in a GROUP BY, the query will return the value from every row in the table. Without the GROUP BY to correlate the two, the DBMS does not know what to do. No wonder some people call them aggravating functions instead of aggregate functions. But it really isn t that dif cult. Just remember one rule: Whenever a query includes an aggregate function, then every column in the query results must either be formed using an aggregate function or be named in the GROUP BY clause.
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