Common SQL Questions - International Informix Users Group

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. 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 SQLrelated
questions. We'll also discuss how to find out how others have found
answers and solutions to similar problems.
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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
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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.
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Date and Time Data types
• DATE (mm/dd/yyyy)
• DATETIME (yyyy-mm-dd hh:mm:ss.nnnnn)
• INTERVAL (year-month OR day-time)
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-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.
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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) = 01/01/2006
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The DATE function will be described in a few slides.
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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)
06-02-28 11:03:27.002
Year Day Minute Fraction
Month Hour Second
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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
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INTERVAL
• Represents a span of time - independent of any
actual date
• Year-Month intervals
YEAR, MONTH
• Day-Time intervals
DAY, HOUR, MINUTE, SECOND, FRACTION
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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.
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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: 2006-02-17 17:37:12.000
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Date/Time Built-in Functions
• DATE
• DAY
• MONTH
• YEAR
• MDY
• EXTEND
• WEEKDAY
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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’)
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See the examples at the beginning of this section where an INTEGER is used
as an argument.
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Example with DATE
• Question: I am trying to select records using a
datetime field. I only care about the date part
when doing my select. Can I select the whole day
record without having to account for all the time
possibilites within a given day?
SELECT * FROM tablename
WHERE DATE(datetime_col) = TODAY - 1;
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DAY
• Returns an integer representing the day of the
month
• DAY(date/datetime expression)
SELECT * FROM orders
WHERE DAY(order_date) > 15
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Search for orders that were placed after the 15th day of any month.
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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) = 12
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Select all from cust_calls where the call was recorded in December (12th
month).
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YEAR
• Returns a four-digit integer that represents the
year
• YEAR(date/datetime expression)
SELECT * FROM orders
WHERE YEAR(paid_date) = YEAR(TODAY)
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Select all orders that were paid in the current year.
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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
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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.
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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: 2006-02-17 00:00
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We are extending the precision of TODAY so that it includes hours and
minutes.
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Example with EXTEND
• Question: I am trying to select records using a
datetime field. I only care about the date part
when doing my select. Can I select the whole day
record without having to account for all the time
possibilites within a given day?
SELECT * FROM tablename
WHERE EXTEND(datetime_col, year to day) = TODAY - 1;
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Note - same question as seen on slide 11, but with a different solution.
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Same question - another way
SELECT * FROM tablename
WHERE EXTEND(datetime_col, year to day) = “2006-05-01”;
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Note: If you are going to enter a string to represent the day that you are
looking for (May 1, 2006 in this example), be sure to format that string
properly for the DATETIME datatype.
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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
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Find all customer calls that came in on a Friday.
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UNITS of time
• YEAR, MONTH, DAY, HOUR, MINUTE, SECOND,
FRACTION
SELECT * FROM call_log
WHERE call_date >= (TODAY - 7 UNITS DAY)
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You can not add, for example, MINUTE UNITS to a DATE -- the UNITS of
time must be compatible with the datatype.
Attempting to add minutes to a date produced the following error (-1266):
Intervals or datetimes are incompatible for the operation.
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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!
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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
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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.
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How do I get the last day of the
current month?
SELECT (CURRENT - (DAY(CURRENT)) UNITS DAY
+ 1 UNITS DAY) + 1 UNITS MONTH - 1 UNITS DAY
FROM systables WHERE tabid = 1;
Also works:
SELECT (CURRENT - (DAY(CURRENT) - 1) UNITS DAY)
+ 1 UNITS MONTH - 1 UNITS DAY
FROM systables WHERE tabid = 1;
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Once you’ve worked out how to get the last day of the current month, it
shouldn’t be too difficult to work out how to get the last day or the next or
previous month.
Although this example used CURRENT (DATETIME data type), handling a
DATE data type is another option - as is using MDY and EXTEND and ….
Remember: TMTOWTDI (There’s more than one way to do it).
SELECT MDY(MONTH(TODAY) + 1, 1, YEAR(TODAY)) - 1
MDY returns a DATE - get first day of next month, and subtract a day
(remember a DATE is stored as an integer - don’t need to include the UNITS
keyword).
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I’m still having trouble with
dates…
• Much of what you will need to do has already been
done before.
• A few more examples…
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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: http://www.catb.org/~esr/faqs/smart-questions.html
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Select n rows - How do you get
just a few?
• Query the database:
SELECT
FROM
WHERE
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But I only want…
• Putting limits on the rows returned from a SELECT
• Select-clause Keywords
• New features in IDS v10.00xC3
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FIRST, MIDDLE, … LAST?
• SKIP offset (IDS v10.00.xC3)
• FIRST max
• LIMIT max (synonym for FIRST - IDSv10.00.xC3)
• MIDDLE max (XPS only)
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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…
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LIMIT, introduced with IDSv10.00xC3 is a synonym for FIRST.
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MIDDLE
• Available with XPS only
• Requires use of a max value
• Syntax and restrictions the same as for FIRST
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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
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User-Defined Routines & SPL
• Distinction between routines and functions
• CURRENT
• Dynamic SPL
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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
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Note - because procedures do not return values, they cannot be used in an SQL
expression.
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Why doesn’t the time change?
• CURRENT
• Returns the date/time the routine was started
• Standard behavior - it isn’t a bug
• SELECT DBINFO(‘utc_to_datetime’, sh_curtime)
FROM sysmaster:sysshmvals
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Can I use dynamic SPL?
• No! Not by default, anyway.
• exec bladelet (IDS v9.x)
• IBM DeveloperWorks
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http://www-
128.ibm.com/developerworks/db2/zones/informix/library/samples/db_downloa
ds.html
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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
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Discussion.
For more, see John Miller III sessions and Chat with the Lab presentations for
a comprehensive discussion.
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Newlines
• How do you remove newlines in text?
• "tr -d '\015'"
• C program
• Others?
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Discussion.
There are many suggested methods of removing newlines from text. There are
a number of solutions provided at comp.databases.informix.
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Resources
• IBM Informix manuals on-line
http://www.ibm.com/software/data/informix/pubs/library/
• IIUG SIGs (Special Interest Groups)
http://www.iiug.org/
• Google
http://groups.google.com/advanced_search
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More help -
• IIUG website - IBM Informix Links
• Smart Questions:
http://www.catb.org/~esr/faqs/smart-questions.html
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Session L04
Common SQL Questions: Why and How-to
June C. Hunt
Tyler Technologies Inc., MUNIS Division
jhunt@munis.com
june@iiug.org
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