Common SQL Questions

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L04 Common SQL Questions Why and How-to Tuesday, May 9, 2006 08:30a.m. 09:40 a.m. There are a few questions that come up time and time again.

1 L04 Common SQL Questions Why and How-to Tuesday, May 9, :30a.m. 09:40 a.m. There are a few questions that come up time and time again. Why do certain SQL statements behave the way they do, and how do I get a particular set of results? This presentation will cover some of the most frequently asked SQL-related questions. We'll also discuss how to find out how others have found answers and solutions to similar problems. 1

2 Agenda Using DATE and DATETIME in calculations Putting limits and order on rows returned User-Defined Routines and SPL considerations Explain plan query estimates Replace newlines in text 2 2

3 How can I manipulate dates and times? Common problems with date and time manipulation are due to misunderstandings about the data types. There are a number of built-in functions that will help. 3 3

4 Date and Time Data types DATE (mm/dd/yyyy) DATETIME (yyyy-mm-dd hh:mm:ss.nnnnn) INTERVAL (year-month OR day-time) 4 -The DBDATE and GL_DATE environment variables may be used to change the default DATE format. -The default precision for DATETIME fraction is 3 digits; may use as many as 5. -Can not combine the two INTERVAL classes. 4

5 DATE Stored internally as an integer Value is equal to the number of days since December 31, 1899 DATE(1) = 01/01/1900 DATE(38717) = 01/01/ The DATE function will be described in a few slides. 5

6 DATETIME Stores an instant in time as a date and time of day Precision defined from a year to a fraction of a second DATETIME YEAR TO FRACTION(3) :03: Year Day Minute Fraction Month Hour Second 6 Note the format of the DATETIME example. -Hyphen is used between the date portion of the DATETIME -Space separates date from time -Colon is used between hour and minutes, and minutes and seconds -Decimal point separates the fraction from seconds 6

7 INTERVAL Represents a span of time - independent of any actual date Year-Month intervals YEAR, MONTH Day-Time intervals DAY, HOUR, MINUTE, SECOND, FRACTION 7 Because the interval is independent of a specific date, you cannot combine the two classes of intervals. As the number of days in a given month (and some years) varies, a single interval value cannot combine months and days. 7

8 Date/Time Keywords TODAY Returns the system date as a DATE value e.g. select TODAY from systables where tabid = 1; returns: 02/17/2006 CURRENT Returns a DATETIME value representing the current moment e.g. select CURRENT from systables where tabid = 1; returns: :37:

9 Date/Time Built-in Functions DATE DAY MONTH YEAR MDY EXTEND WEEKDAY 9 To do: Include examples for each function. Also include examples of mixing the results of the functions with other mathematical operations. Pay particular attention to the EXTEND function. 9

10 DATE Returns a date value that corresponds to a nondate expression DATE(non-date expression) SELECT * FROM orders WHERE order_date > DATE( 05/01/2006 ) 10 See the examples at the beginning of this section where an INTEGER is used as an argument. 10

11 DAY Returns an integer representing the day of the month DAY(date/datetime expression) SELECT * FROM orders WHERE DAY(order_date) > Search for orders that were placed after the 15th day of any month. 11

12 MONTH Returns an integer representing the month portion of a DATE or DATETIME MONTH(date/datetime expression) SELECT * FROM cust_calls WHERE MONTH(call_dtime) = Select all from cust_calls where the call was recorded in December (12th month). 12

13 YEAR Returns a four-digit integer that represents the year YEAR(date/datetime expression) SELECT * FROM orders WHERE YEAR(paid_date) = YEAR(TODAY) 13 Select all orders that were paid in the current year. 13

14 MDY Returns a DATE value with three expressions that evaluate to integers that represent the month, day, and year MDY(month,day,year) UPDATE orders SET ship_date = MDY(MONTH(TODAY) + 1, 1, YEAR(TODAY)) WHERE order_date = TODAY 14 Update the orders table, setting the ship date to the first day of next month (based on today s date) for all orders placed today. 14

15 EXTEND Adjusts the precision of a DATETIME or DATE value EXTEND(date/datetime expression, first TO last) SELECT EXTEND(TODAY, YEAR TO MINUTE) FROM systables WHERE tabid = 1; Returns: :00 15 We are extending the precision of TODAY so that it includes hours and minutes. 15

16 WEEKDAY Returns an integer that represents a day of the week (0 = Sunday, 6 = Saturday) WEEKDAY(date/datetime expression) SELECT * FROM cust_calls WHERE WEEKDAY(call_dtime) = 5 16 Find all customer calls that came in on a Friday. 16

17 UNITS of time YEAR, MONTH, DAY, HOUR, MINUTE, SECOND, FRACTION SELECT * FROM call_log WHERE call_date >= (TODAY - 7 UNITS DAY) 17 17

18 Adding and Subtracting time Why do I get an error when I subtract a month? Example: Let s say today is 03/30/2005 Today - 1 UNITS MONTH = ERROR 02/30/2005 is an invalid date! 18 18

19 How do I get the first day of the previous or next month? SELECT order_num, order_date, DATE(EXTEND(order_date, YEAR TO MONTH) - 1 UNITS MONTH) AS prev_month, DATE(EXTEND(order_date, YEAR TO MONTH) + 1 UNITS MONTH) AS next_month FROM orders 19 In this example, we are adjusting the precision of the order date so that we are taking just the year and month and using that as an argument to the DATE function. In addition, we are either subtracting or adding one month to the result of the EXTEND. The final result will be to give us the order number, the original order date, the first of the month prior to the order date, and the first of the month following the order date. 19

20 I m still having trouble with dates Much of what you will need to do has already been done before. A few more examples 20 If you are unable to work it out through testing, and documentation and Google searches aren t turning up what you need, don t hesitate to ask the community. Most of us enjoy a challenge, but please remember to do your homework first. If you haven t already read the Smart Questions FAQ, it will help you to pose a question that will get you the best possible answer - See: 20

21 Select n rows - How do you get just a few? Query the database: SELECT <select clause> FROM <data source> WHERE <condition> 21 21

22 But I only want Putting limits on the rows returned from a SELECT Select-clause Keywords New features in IDS v10.00xc

23 FIRST, MIDDLE, LAST? SKIP offset (IDS v10.00.xc3) FIRST max LIMIT max (synonym for FIRST - IDSv10.00.xC3) MIDDLE max (XPS only) 23 23

24 FIRST/LIMIT Requires use of a max value SELECT FIRST 10 sales_reps, total_sales FROM sales ORDER BY total_sales DESC Restrictions View Nested SELECT Subqueries Others 24 LIMIT, introduced with IDSv10.00xC3 is a synonym for FIRST. 24

25 MIDDLE Available with XPS only Requires use of a max value Syntax and restrictions the same as for FIRST 25 25

26 SKIP Requires use of an offset value SELECT SKIP 50 col1, col2 FROM table1 SELECT SKIP 50 FIRST 10 col1, col2 FROM table1 Restrictions similar to FIRST 26 26

27 User-Defined Routines & SPL Distinction between routines and functions CURRENT Dynamic SPL 27 27

28 What s the difference? Procedures May accept arguments Does not return values Can not be used in SQL expressions Functions May accept arguments Returns one or more values Can be used in SQL expressions 28 Note - because procedures do not return values, they cannot be used in an SQL expression. 28

29 Why doesn t the time change? CURRENT Returns the date/time the routine was started Standard behavior - it isn t a bug 29 29

30 Can I use dynamic SPL? No! Not be default, anyway. exec bladelet (IDS v9.x) IBM DeveloperWorks ibm.com/developerworks/db2/zones/informix/library/samples/db_downloads.ht ml 30

31 Query Estimates What does that query estimate really mean? Compare estimates between modified versions of the same statement How is the query estimate determined? It depends on who you ask 31 Discussion. For more, see John Miller III sessions and Chat with the Lab presentations for a comprehensive discussion. 31

32 Newlines How do you remove newlines in text? "tr -d '\015'" C program Others? 32 Discussion. There are many suggested methods of removing newlines from text. There are a number of solutions provided at comp.databases.informix. 32

33 Resources IBM Informix manuals on-line IIUG SIGs (Special Interest Groups) Google

34 Session L04 Common SQL Questions: Why and How-to June C. Hunt Tyler Technologies Inc., MUNIS Division

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