Advanced SELECT Queries CS 146

Review: Retrieving Data From a Single Table Syntax: Limitation: Retrieves "raw" data Note the default formats… SELECT column1, column2, … FROM tablename WHERE search_condition(s)

We can use the DBMS to manipulate retrieved data!  Suppress duplicates  Sort  Format characters, numbers, & dates  Perform arithmetic operations  Summarize groups of data  Why not just do it in your program?

Suppressing Duplicate Outputs  Use the DISTINCT qualifier  Ensures that only distinct rows are returned SELECT DISTINCT cust_zip FROM candy_customer;

Sorting Query Output Use the ORDER BY clause: Always appears as the last item in a SELECT query SELECT custname FROM customer WHERE cust_type = 'P' ORDER BY cust_name;

Sorting Query Output  Default sort order is ascending  Numbers: smallest to largest  Characters: alphabetical  Dates: oldest to newest  To force a descending sort order, add the DESC modifier: SELECT purch_id, purch_date FROM candy_purchase ORDER BY purch_date DESC

Multiple Sort Keys  You can sort output by multiple keys  Only makes sense when first sort key has repeating values… SELECT purch_id, purch_date FROM candy_purchase ORDER BY purch_date DESC, purch_id

Formatting Number Output  Use the FORMAT function  Format: FORMAT(number, decimal_places) SELECT purch_id, FORMAT(pounds, 2) FROM candy_purchase;

Formatting Number Output as Currency  Use the CONCAT and FORMAT function  CONCAT joins two strings to create a single string  CONCAT('$', FORMAT(number, decimal_places)) SELECT prod_id, CONCAT('$', FORMAT(prod_cost, 2)) FROM candy_product;

Formatting Date Output  Use the DATE_FORMAT function  Format: DATE_FORMAT(date, 'format') SELECT purch_id, DATE_FORMAT(purch_date, '%b %e, %Y') FROM candy_purchase; Predefined format specifiers

Format Specifiers  %b – abbreviated month name  %e – day of the month, numeric, suppresses leading zeroes  %Y – year, numeric, 4 digits  More complete list of specifiers More complete list of specifiers

Formatting Character Output  MySQL has a variety of functions for manipulating strings FunctionDescriptionExample Query CONCAT(, )Concatenates (joins) two strings SELECT CONCAT(cust_addr, cust_zip) FROM candy_customer; UPPER( ), LOWER( ) Returns the string, with all characters converted to upper/lower case SELECT UPPER(username) FROM candy_customer; LENGTH( )Returns an integer representing the string length SELECT LENGTH(password) FROM candy_customer; LPAD(,, ), RPAD(,, ) Returns the value of the string, with sufficient padding characters added to the left/right edge so return value equals total length specified SELECT LPAD(password, 8, '*') FROM candy_customer;

Default Query Output  Column names are database field names  Calculated column names are the formula

Column Aliases  Provide an alternate column name  What good are they?  You can use them in the ORDER BY clause  You can reference them in embedded programs  NOTE: alias only has to be in single quotes if it contains blank spaces SELECT LENGTH(cust_name) AS 'name length' FROM candy_customer ORDER BY 'name length';

Performing Arithmetic Calculations in Queries  Applications often perform arithmetic operations on retrieved data  You can perform basic arithmetic operations on numbers and dates in a SQL query SELECT clause  Rationale:  DBMS makes it easy to perform the operation  Network needs to transmit only the data you need

Performing Arithmetic Operations on Number Data  Operators: +, -, *, /  Order of evaluation:  * / then + -  To force a different order, use parentheses  Only use on number data  Prices, quantities, etc.

Performing Arithmetic Operations on Number Data  Example: SELECT prod_desc, prod_price - prod_cost FROM candy_product;

Performing Arithmetic Operations on Date Data  To display a date that is a specific number of days after/before a stored date, add/subtract the number of days: SELECT purch_id, purch_date, purch_date + 2 FROM candy_purchase; SELECT purch_id, purch_date, purch_date - 2 FROM candy_purchase;

Performing Arithmetic Operations on Date Data  To calculate the number of days between two known dates, use DATEDIFF SELECT purch_id, purch_date, delivery_date, DATEDIFF(delivery_date, purch_date) FROM candy_purchase;

Retrieving the Current Date  Use the CURRENT_DATE() function  Function: code retrieving information that acts like a column in a SQL command SELECT CURRENT_DATE();

Another Date Calculation  Calculating someone’s age from their date of birth SELECT (DATEDIFF(CURRENT_DATE(), ' ')) / ;

SQL Group Functions  Performs an operation on a field from a group of retrieved records  AVG (average of all retrieved values)  COUNT (number of records retrieved)  MAX (maximum value retrieved)  MIN (minimum value retrieved)  SUM (sum of all retrieved values)

SQL Group Functions  Examples SELECT MAX(prod_cost), MIN(prod_cost), AVG(prod_cost) FROM candy_product; SELECT COUNT(*) FROM candy_customer;

Using the GROUP BY Clause  Whenever you use a group function:  All of the columns in the select clause must be in a group function  or  If not, the column must be listed in a GROUP BY clause

Using the GROUP BY Clause  Example: SELECT purch_date, MAX(pounds), MIN(pounds), AVG(pounds) FROM candy_purchase GROUP BY purch_date;

SUM and Statistical Functions  SUM, AVG, MAX, MIN  Can only be used with NUMBER columns SUM(pounds) MAX(prod_cost) MIN(prod_cost) AVG(prod_cost)

COUNT Function  Displays the number of records that a query will retrieve  Can be used on a column of any data type  Forms:  COUNT(*) – displays total number of records, regardless if the record has fields that contain NULL values  COUNT(fieldname) – displays the number of retrieved records in which the specified field is NOT NULL