SQL*Plus Substitution Variables

This tutorial from the Oracle Technology Network outline the use of the three types of variables available in SQLPlus. Bind variables, substitution variables and system variables are all explained in this succinct yet thorough tutorial. The best I’ve seen on the subject.

SQL*Plus Substitution Variables from Oracle Technology Network.

For information on other Oracle web resources, check out my other article on the topic.

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Converting Time Zones in Oracle

In my previous article Oracle, SQL, Dates and Timestamps I talked about inserting, selecting and comparing dates in Oracle. Now I want to cover some functionality for converting between time zones.

We’ll use the table in the form specified in the previous article, but insert one more row:

insert into dates values(6, to_date('09/20/05 23:15', 'MM/DD/YY HH24:MI'));

The contents of the table now look like this:

1 09/14/05, 21:08
2 09/27/05, 00:00
3 10/02/05, 22:05
4 09/01/05, 17:01
5 09/12/05, 14:30
6 09/20/05, 23:15

Changing Time Zones

The date format in Oracle does not contain time zone information, but the database does. To find out the time zone set, execute this query:

SELECT dbtimezone FROM dual;

DBTIME
------
-04:00

The time zone can be updated with the command:

ALTER database SET TIME_ZONE = '-05:00';

where you can specify the offset from Greenwich mean time or a valid time zone from the list in the v$timezone_names view. Note that this is one of the few of the ‘v$’ views which are plural.

Switching Time Zones

The function new_time is used to convert a time to different time zones. To illustrate this we’ll look at entry 5 from the dates file.

SELECT entry, to_char(entry_date, 'MM/DD/YY HH:MI AM') FROM dates WHERE entry=5;

5 09/12/05 02:30 PM

This database is in US Eastern time but we want to display the time in US Central.

SELECT entry, to_char(new_time(entry_date, 'EST', 'CST'), 'MM/DD/YY HH:MI AM') FROM dates WHERE entry=5;

5 09/12/05 01:30 PM

Here we clearly see the time converted to Central. Note that the new_time function is performed on the date field, not on the to_char. Now let’s grab this time in Pacific time:

SELECT entry, to_char(new_time(entry_date, 'EST', 'PST'), 'MM/DD/YY HH:MI AM') FROM dates WHERE entry=5;

5 09/12/05 11:30 AM

Now we see not only the time converted, but also the time of day has gone from PM to AM.

Now let’s take a look at entry 6:

SELECT entry, to_char(entry_date, 'MM/DD/YY HH:MI AM') FROM dates WHERE entry=6;

6 09/20/05 11:15 PM

We’ll again assume this timestamp is in US Eastern time, but let’s convert it this time to Greenwich Mean Time.

SELECT entry, to_char(new_time(entry_date, 'EST', 'GMT'), 'MM/DD/YY HH:MI AM') FROM dates WHERE entry=6;

6 09/21/05 04:15 AM

This shows not only the change in hours, but that the date of this entry is displayed properly for its time zone.

Of course the new_time function can be used on inserts in the same way. This is useful if you are allowing input from people in different geographical regions. Here we convert an entry made in Pacific Time to Eastern:

INSERT INTO dates
VALUES (7,
new_time(to_date('09/22/05 10:28 AM', 'MM/DD/YY HH:MI AM'), 'PST', 'EST'));

SELECT entry, to_char(entry_date, 'MM/DD/YY HH:MI AM') FROM dates WHERE entry=7;

7 09/22/05 01:28 PM

So we have converted 10:28 AM Pacific to 1:28 PM Eastern so all our entries in the table are consistent. Of course when performing the insert we need to put the to_date function within the new_time function so the text string is converted to a date format before we try to convert it.

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SQL Join Syntax

I have always found the different types of joins a bit confusing, but now thanks to a little experimenting I think I have a handle on it. This is geared toward Oracle, but most of this is ANSI SQL, so should work in other databases as well. Some of these features may not be available in Oracle pre-9i.

Assume these two tables:

SELECT * FROM faculty;

ID FIRST_NAME LAST_NAME
1 Jon Emmons
2 Zach Tirrell
3 Evelyn Stiller

SELECT * FROM class;

CLASS_ID CATALOG_NUMBER INSTRUCTOR_ID
1 CS3600 1
2 CS3020 2
3 CS2000  
4 CS1100 1

Inner Join

The most common (and simple) join. This will select the rows which satisfy the join condition; however if a row exists in one table but does not have a counterpart to fulfill the join condition those rows (from either table) will be ignored.

SELECT *
FROM faculty, class
WHERE faculty.id = class.instructor_id;

ID FIRST_NAME LAST_NAME CLASS_ID CATALOG_NUMBER INSTRUCTOR_ID
1 Jon Emmons 1 CS3600 1
2 Zach Tirrell 2 CS3020 2
1 Jon Emmons 4 CS1100 1

Here we see the three rows where faculty.id had a match to class.instructor_id.

Left Join

A left join, a.k.a. left outer join will return all the row combinations which meet the join condition plus any rows from the first table which do not meet the guard condition.

SELECT *
FROM faculty LEFT JOIN class
ON faculty.id=class.instructor_id;

ID FIRST_NAME LAST_NAME CLASS_ID CATALOG_NUMBER INSTRUCTOR_ID
1 Jon Emmons 1 CS3600 1
1 Jon Emmons 4 CS1100 1
2 Zach Tirrell 2 CS3020 2
3 Evelyn Stiller      

Now we see all the results we saw in the inner join, but we additionally see the row from faculty (id 3, Evelyn Stiller) which has no corresponding row in class.

Right Join

Also referred to as right outer join, this will show all row combinations which meet the join criteria, but will additionally show any rows from the second table which do not have counterparts in the first.

SELECT *
FROM faculty RIGHT JOIN class
ON faculty.id=class.instructor_id;

ID FIRST_NAME LAST_NAME CLASS_ID CATALOG_NUMBER INSTRUCTOR_ID
1 Jon Emmons 1 CS3600 1
2 Zach Tirrell 2 CS3020 2
      3 CS2000  
1 Jon Emmons 4 CS1100 1

Now we see rows from the class table which do not have a faculty counterpart.

Outer Join

The outer join, or full outer join can be thought of a left join and right join. Rows which meet the join condition will of course be displayed, additionally, rows from both the first and second table referenced will be displayed.

SELECT *
FROM faculty FULL OUTER JOIN class
ON faculty.id = class.instructor_id;

ID FIRST_NAME LAST_NAME CLASS_ID CATALOG_NUMBER INSTRUCTOR_ID
1 Jon Emmons 1 CS3600 1
1 Jon Emmons 4 CS1100 1
2 Zach Tirrell 2 CS3020 2
3 Evelyn Stiller      
      3 CS2000  

Now we see all the rows from both the tables, joined where the join condition is met, or with corresponding null values where the join condition failed.

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Oracle, SQL, Dates and Timestamps

A common question amongst database developers is “How do I get dates and time into and out of the database in the format my script/program/table expects?” This information is based on Oracle, however I expect much of this will apply to other databases.

About the DATE and TIMESTAMP datatypes: The DATE datatype is 7-bytes, comprised of date and time information to the precision of 1 second.
TIMESTAMP can be from 7 to 11 bytes depending on the precision specified. Timestamps can represent date and time as small as the nanosecond (.000000001 seconds) The default is to a microsecond of precision (.000001 seconds.)

Note: dual is a special table for testing and development. It’s useful for returning values (results from functions or contents of variables) not stored in tables, for instance the current date.

Let’s start with some information we can grab from the system about the current date and time.

SELECT sysdate FROM dual;

SYSDATE
---------
14-SEP-05


This simple select statement returns the date in the standard format (typically DD-MON-YY.)

SELECT systimestamp FROM DUAL;

SYSTIMESTAMP
---------------------------------------------------------------------------
14-SEP-05 04.06.31.264201 PM -04:00

Here we see that systimestamp reports more detail than sysdate, including the offset from GMT.

Displaying Dates

The to_char function will allow you to describe how you want dates displayed and will convert them to a character string in that format. The default in Oracle is DD-MON-YY. The default format can be changed by setting the nls_date_format parameter.

SELECT to_char(sysdate, 'MM/DD/YYYY') FROM dual;

TO_CHAR(SY
----------
09/14/2005

As seen here, the to_char function requires two parameters: a date to display, and the format you want it to be in. There are dozens of formatting options, but here are some common ones:

SELECT to_char(sysdate, 'MM/DD/YY') FROM dual;

09/14/05

SELECT to_char(sysdate, 'MM/DD/YYYY HH:MI:SS') FROM dual;

09/14/2005 04:09:03

SELECT to_char(sysdate, 'DAY, MONTH DD, HH12:MI AM') FROM dual;

WEDNESDAY, SEPTEMBER 14, 04:09 PM

SELECT to_char(sysdate, 'YYYY BC') FROM dual;

2005 AD

SELECT to_char(systimestamp, 'HH24:MI:SS.FF3') FROM dual;

16:09:24.606

There are several more options than are displayed here. Your databases documentation should have a full list. These components can be used in any order or combination. These characters, as well as spaces can be used to format dates / . – : . ;

Inserting Dates

to_date works similarly to the to_char function above. You must specify a date, typically enclosed by single quotes, then describe the format with the date components as above. To demonstrate this we’ll create a table we can insert some dates into.

CREATE TABLE dates
(
entry NUMBER,
entry_date DATE,
CONSTRAINT pk_dates PRIMARY KEY (entry)
);

Now a few inserts:

INSERT INTO dates (entry, entry_date)
VALUES (1, sysdate);

Inserts the current date and time to the second.

INSERT INTO dates (entry, entry_date)
VALUES (2, to_date('09/27/05', 'MM/DD/YY'));

INSERT INTO dates (entry, entry_date)
VALUES (3, to_date('10/02/2005 10:05:33 PM', 'MM/DD/YYYY HH:MI:SS AM'));

INSERT INTO dates (entry, entry_date)
VALUES (4, to_date('17:01:24', 'HH24:MI:SS'));

INSERT INTO dates (entry, entry_date)
VALUES (5, to_date('Monday, September 12, 2:30 PM', 'DAY, MONTH DD, HH:MI AM'));

COMMIT;

Now let’s take a look at the data in the dates table:

SELECT entry, to_char(entry_date, 'MM/DD/YYYY HH:MI:SS AM')
FROM dates;

1 09/14/2005 09:08:32 PM
2 09/27/2005 12:00:00 AM
3 10/02/2005 10:05:33 PM
4 09/01/2005 05:01:24 PM
5 09/12/2005 02:30:00 PM

We can see that the current date and time was entered in entry 1 down to the second.

Entry 2 contains the date we entered, but since we did not specify the time it has defaulted to midnight.

Entry 3 shows a complete timestamp exactly as we specified.

In entry 4 we see the time as we specified, but since we didn’t specify a date it has defaulted to the first of this month. I have a feeling this varies from database to database. Probably best not to rely on this.

Entry 5 shows the date and time, however since we did not specify seconds they display as :00.

Comparing Dates

Dates can be compared much like other values. To demonstrate this we’ll do some quick selects on the table we just created.

select entry, to_char(entry_date, 'MM/DD/YYYY HH:MI:SS AM')
from dates
where entry_date > to_date('09/20/2005', 'MM/DD/YYYY');

2 09/27/2005 12:00:00 AM
3 10/02/2005 10:05:33 PM

select entry, to_char(entry_date, 'MM/DD/YYYY HH:MI:SS AM')
from dates
where entry_date < sysdate; 1 09/14/2005 09:08:32 PM 4 09/01/2005 05:01:24 PM 5 09/12/2005 02:30:00 PM select entry, to_char(entry_date, 'MM/DD/YYYY HH:MI:SS AM') from dates order by entry_date; 4 09/01/2005 05:01:24 PM 5 09/12/2005 02:30:00 PM 1 09/14/2005 09:08:32 PM 2 09/27/2005 12:00:00 AM 3 10/02/2005 10:05:33 PM

Those are the highlights. Most things you'll need to do will be some type of variation on these.

oracle, sql, dba, database administration, database development

Oracle Optimal Flexible Architecture

Originally written in 1990, the Optimal Flexible Architecture (OFA) whitepaper still stands as the best-practices for oracle databases. In my time as an Oracle database administrator I have often seen DBAs using these standards, having learned them from senior DBAs, who did not realize the OFA standard exists.

Cary V. Millsap of Oracle Corp. offers this description in this distribution of the whitepaper:

The OFA Standard is a set of configuration guidelines that will give you faster, more reliable Oracle databases that require less work to maintain. The OFA Standard is written by the founder of the Oracle team responsible for installing, tuning, and upgrading several hundreds of sites worldwide since 1990—this paper is based on the best practices of those hundreds of sites. Today the “Optimal Flexible Architecture’’ described in the OFA Standard is built into the Oracle configuration tools and documentation on all open systems ports.

The benifits of the OFA standard go beyond performance and stability. When I started at Plymouth State University 18 months ago I quickly recognized the signs of an OFA environment (mostly partitions named /u01 and /u02.) Having identified that, I immediately knew where to find nearly all data and configuration files.

The guidelines that OFA provides can be easily adapted to other modern multi-user applications including web servers, application servers and other breeds of database. I offer the OFA whitepaper here as I have found it increasingly difficult to find on the web.

The OFA Standard-Oracle for Open Systems

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