what is SQL?
Structured Query Lanaguage.
What is SELECT Statement?
The select statement lets you select a set of values from a table in a database.The values selected from the database table would depend on the various condition that are specified in the SQL query.
How can you compare a part of the name rather than the entire name ?
SELECT * FROM people WHERE empname like '%ab%'.
This returns a recordset with records consisting empname the sequence 'ab'.
What is INSERT Statement?
INSERT statement lets you insert information into a database.
How do you delete a record from a database?
Use delete statement to remove records or any particular column values from a database.
How could i get distinct entries from a table?
The select statement in conjunction with DISTINCT lets you select a set of distinct values from a table in a database.The values selected from the database table would of course depend on the various conditions that are specified in the SQL query.
Example:
SELECT DISTINCT empname FROM emptable.
How to get the results of a query sorted in any order?
You can sort the results and return the so
ORDER BY keyword thus saving you the pain of carrying out the sorting yourself.The ORDER BY keyword is used for sorting.
SELECT empname,age city FROM emptable ORDER BY empname
How can i find the total number of records in a table ?
Use COUNT(*)
SELECT COUNT(*) FROM emp WHERE age>40
What is GROUP BY?
The GROUP BY keywords have been added to SQL because aggregate functions (like SUM) return the aggregate of all column values every time they are called.Without the GROUP BY functionality, finding the sum for each individual group of column values was not possible.
what is the difference among "dropping a table","truncation a table" and "deleting all records" from a table ?
DROPPING[Table Structure+Data ]: Invalidates the dependent objects,drops the indexes.
TRUNCATING[Data]:Performs an automatic commit,faster than delete.
DELETE[Data]:Does not perform automatic commit.
What are the Large object types supported by oracle?
BLOB & CLOB
Difference between a "where" clause and "having" clause?
Having clause is used only with group functions .
what is the difference between a primary key and a unique key?
Both primary key and unique key enforce uniqueness of the column on which they are defined.But by default primary key creates a clustered index on the column, whereas unique key creates a non-clustered index by default.Another major difference is that primary key does not allow NULL,but unique key allows one NULL only.
What are cursors?
Cursors allow row-by-row processing of the resultsets.
Types :
1.Static
2.Dynamic
3.Forward-Only
4.Keyset-driven.
Disadvantages of Cursors:
Each time you fetch a row from the cursor,it results in a network roundtrip,where as a normal SELECT query makes only one roundtrip,however large the resultset is.
Cursors are also costly because they require more resources and temporary storage(results in more IO operations).Further,there are restrictions on the SELECT statements that can be used with some types of cursors.
Most of the times,set based operations can be used instead of cursors.
What are triggers?How to invoke a trigger on demand?
Triggers are special kind of strored procedures that get executed automatically when an INSERT,UPDATE or DELETE operation takes place on a table.
Triggers cant be invoked on demand.They get triggered only when an associated action (INSERT,UPDATE,DELETE) happens on the table on which they are defined.
Triggers are generally used to implement business rules,auditing.Triggers can also be used to extend the referential integrity checks,but wherever possible,use constraints for this purpose,intead of triggers, as constraints are much faster.
What is a join ?
Joins are used in queries to explain how different tables are related.Joins also let you select data from a table depending upon data from another table.
Types:
1.Inner Join
2.Outer Join
3.Cross Join
Outer Join is further classfied as
1.Left Outer Join
2.Right Outer Join
3.Full Outer Join.
What is Self-Join?
Self-Join is just like any other join,except that two instances of the same table will be joined in the query.
What is PL/SQL and what is it used for?
PL/SQL is Oracle's Procedural Language extension to SQL. PL/SQL's language syntax, structure and data types are similar to that of ADA. The PL/SQL language includes object oriented programming techniques such as encapsulation, function overloading, information hiding (all but inheritance). PL/SQL is commonly used to write data-centric programs to manipulate data in an Oracle database.
Should one use PL/SQL or Java to code procedures and triggers?
Internally the Oracle database supports two procedural languages, namely PL/SQL and Java. This leads to questions like "Which of the two is the best?" and "Will Oracle ever desupport PL/SQL in favour of Java?".
Many Oracle applications are based on PL/SQL and it would be difficult of Oracle to ever desupport PL/SQL. In fact, all indications are that PL/SQL still has a bright future ahead of it. Many enhancements are still being made to PL/SQL. For example, Oracle 9iDB supports native compilation of Pl/SQL code to binaries.
PL/SQL and Java appeal to different people in different job roles. The following table briefly describes the difference between these two language environments:
PL/SQL:
Data centric and tightly integrated into the database
Proprietary to Oracle and difficult to port to other database systems
Data manipulation is slightly faster in PL/SQL than in Java
Easier to use than Java (depending on your background)
Java:
Open standard, not proprietary to Oracle
Incurs some data conversion overhead between the Database and Java type systems
Java is more difficult to use (depending on your background)
How can one see if somebody modified any code?
Code for stored procedures, functions and packages is stored in the Oracle Data Dictionary. One can detect code changes by looking at the LAST_DDL_TIME column in the USER_OBJECTS dictionary view. Example:
SELECT OBJECT_NAME,
TO_CHAR(CREATED, 'DD-Mon-RR HH24:MI') CREATE_TIME,
TO_CHAR(LAST_DDL_TIME, 'DD-Mon-RR HH24:MI') MOD_TIME,
STATUS
FROM USER_OBJECTS
WHERE LAST_DDL_TIME > '&CHECK_FROM_DATE';
How can one search PL/SQL code for a string/ key value?
The following query is handy if you want to know where a certain table, field or _expression is referenced in your PL/SQL source code.
SELECT TYPE, NAME, LINE
FROM USER_SOURCE
WHERE UPPER(TEXT) LIKE '%&KEYWORD%';
How can one keep a history of PL/SQL code changes?
One can build a history of PL/SQL code changes by setting up an AFTER CREATE schema (or database) level trigger (available from Oracle 8.1.7). This way one can easily revert to previous code should someone make any catastrophic changes. Look at this example:
CREATE TABLE SOURCE_HIST -- Create history table
AS SELECT SYSDATE CHANGE_DATE, USER_SOURCE.*
FROM USER_SOURCE WHERE 1=2;
CREATE OR REPLACE TRIGGER change_hist -- Store code in hist table
AFTER CREATE ON SCOTT.SCHEMA -- Change SCOTT to your schema name
DECLARE
BEGIN
if DICTIONARY_OBJ_TYPE in ('PROCEDURE', 'FUNCTION',
'PACKAGE', 'PACKAGE BODY', 'TYPE') then
-- Store old code in SOURCE_HIST table
INSERT INTO SOURCE_HIST
SELECT sysdate, user_source.* FROM USER_SOURCE
WHERE TYPE = DICTIONARY_OBJ_TYPE
AND NAME = DICTIONARY_OBJ_NAME;
end if;
EXCEPTION
WHEN OTHERS THEN
raise_application_error(-20000, SQLERRM);
END;
/
show errors
How can I protect my PL/SQL source code?
PL/SQL V2.2, available with Oracle7.2, implements a binary wrapper for PL/SQL programs to protect the source code.
This is done via a standalone utility that transforms the PL/SQL source code into portable binary object code (somewhat larger than the original). This way you can distribute software without having to worry about exposing your proprietary algorithms and methods. SQL*Plus and SQL*DBA will still understand and know how to execute such scripts. Just be careful, there is no "decode" command available.
The syntax is:
wrap iname=myscript.sql onfiltered=xxxx.plb
Can one print to the screen from PL/SQL?
One can use the DBMS_OUTPUT package to write information to an output buffer. This buffer can be displayed on the screen from SQL*Plus if you issue the SET SERVEROUTPUT ON; command. For example:
set serveroutput on
begin
dbms_output.put_line('Look Ma, I can print from PL/SQL!!!');
end;
/
DBMS_OUTPUT is useful for debugging PL/SQL programs. However, if you print too much, the output buffer will overflow. In that case, set the buffer size to a larger value, eg.: set serveroutput on size 200000
If you forget to set serveroutput on type SET SERVEROUTPUT ON once you remember, and then EXEC NULL;. If you haven't cleared the DBMS_OUTPUT buffer with the disable or enable procedure, SQL*Plus will display the entire contents of the buffer when it executes this dummy PL/SQL block.
Can one read/write files from PL/SQL?
Included in Oracle 7.3 is an UTL_FILE package that can read and write operating system files. The directory you intend writing to has to be in your INIT.ORA file (see UTL_FILE_DIR=... parameter). Before Oracle 7.3 the only means of writing a file was to use DBMS_OUTPUT with the SQL*Plus SPOOL command.
Copy this example to get started:
DECLARE
fileHandler UTL_FILE.FILE_TYPE;
BEGIN
fileHandler := UTL_FILE.FOPEN('/tmp', 'myfile', 'w');
UTL_FILE.PUTF(fileHandler, 'Look ma, I''m writing to a file!!!\n');
UTL_FILE.FCLOSE(fileHandler);
EXCEPTION
WHEN utl_file.invalid_path THEN
raise_application_error(-20000, 'ERROR: Invalid path for file or path not in INIT.ORA.');
END;
/
Can one call DDL statements from PL/SQL?
One can call DDL statements like CREATE, DROP, TRUNCATE, etc. from PL/SQL by using the "EXECUTE IMMEDATE" statement. Users running Oracle versions below 8i can look at the DBMS_SQL package (see FAQ about Dynamic SQL).
begin
EXECUTE IMMEDIATE 'CREATE TABLE X(A DATE)';
end;
NOTE: The DDL statement in quotes should not be terminated with a semicolon.
Can one use dynamic SQL statements from PL/SQL?
Starting from Oracle8i one can use the "EXECUTE IMMEDIATE" statement to execute dynamic SQL and PL/SQL statements (statements created at run-time). Look at these examples. Note that statements are NOT terminated by semicolons:
EXECUTE IMMEDIATE 'CREATE TABLE x (a NUMBER)';
-- Using bind variables...
sql_stmt := 'INSERT INTO dept VALUES (:1, :2, :3)';
EXECUTE IMMEDIATE sql_stmt USING dept_id, dept_name, location;
-- Returning a cursor...
sql_stmt := 'SELECT * FROM emp WHERE empno = :id';
EXECUTE IMMEDIATE sql_stmt INTO emp_rec USING emp_id;
One can also use the older DBMS_SQL package (V2.1 and above) to execute dynamic statements. Look at these examples:
CREATE OR REPLACE PROCEDURE DYNSQL AS
cur integer;
rc integer;
BEGIN
cur := DBMS_SQL.OPEN_CURSOR;
DBMS_SQL.PARSE(cur, 'CREATE TABLE X (Y DATE)', DBMS_SQL.NATIVE);
rc := DBMS_SQL.EXECUTE(cur);
DBMS_SQL.CLOSE_CURSOR(cur);
END;
/
More complex DBMS_SQL example using bind variables:
CREATE OR REPLACE PROCEDURE DEPARTMENTS(NO IN DEPT.DEPTNO%TYPE) AS
v_cursor integer;
v_dname char(20);
v_rows integer;
BEGIN
v_cursor := DBMS_SQL.OPEN_CURSOR;
DBMS_SQL.PARSE(v_cursor, 'select dname from dept where deptno > :x', DBMS_SQL.V7);
DBMS_SQL.BIND_VARIABLE(v_cursor, ':x', no);
DBMS_SQL.DEFINE_COLUMN_CHAR(v_cursor, 1, v_dname, 20);
v_rows := DBMS_SQL.EXECUTE(v_cursor);
loop
if DBMS_SQL.FETCH_ROWS(v_cursor) = 0 then
exit;
end if;
DBMS_SQL.COLUMN_VALUE_CHAR(v_cursor, 1, v_dname);
DBMS_OUTPUT.PUT_LINE('Deptartment name: '||v_dname);
end loop;
DBMS_SQL.CLOSE_CURSOR(v_cursor);
EXCEPTION
when others then
DBMS_SQL.CLOSE_CURSOR(v_cursor);
raise_application_error(-20000, 'Unknown Exception Raised: '||sqlcode||' '||sqlerrm);
END;
/
What is the difference between %TYPE and %ROWTYPE?
The %TYPE and %ROWTYPE constructs provide data independence, reduces maintenance costs, and allows programs to adapt as the database changes to meet new business needs.
%ROWTYPE is used to declare a record with the same types as found in the specified database table, view or cursor. Example:
DECLARE
v_EmpRecord emp%ROWTYPE;
%TYPE is used to declare a field with the same type as that of a specified table's column. Example:
DECLARE
v_EmpNo emp.empno%TYPE;
What is the result of comparing NULL with NULL?
NULL is neither equal to NULL, nor it is not equal to NULL. Any comparison to NULL is evaluated to NULL. Look at this code example to convince yourself.
declare
a number := NULL;
b number := NULL;
begin
if a=b then
dbms_output.put_line('True, NULL = NULL');
elsif ab then
dbms_output.put_line('False, NULL NULL');
else
dbms_output.put_line('Undefined NULL is neither = nor to NULL');
end if;
end;
How does one get the value of a sequence into a PL/SQL variable?
As you might know, one cannot use sequences directly from PL/SQL. Oracle (for some silly reason) prohibits this:
i := sq_sequence.NEXTVAL;
However, one can use embedded SQL statements to obtain sequence values:
select sq_sequence.NEXTVAL into :i from dual;
Can one execute an operating system command from PL/SQL?
There is no direct way to execute operating system commands from PL/SQL in Oracle7. However, one can write an external program (using one of the precompiler languages, OCI or Perl with Oracle access modules) to act as a listener on a database pipe (SYS.DBMS_PIPE). Your PL/SQL program then put requests to run commands in the pipe, the listener picks it up and run the requests. Results are passed back on a different database pipe. For an Pro*C example, see chapter 8 of the Oracle Application Developers Guide.
In Oracle8 one can call external 3GL code in a dynamically linked library (DLL or shared object). One just write a library in C/ C++ to do whatever is required. Defining this C/C++ function to PL/SQL makes it executable.
How does one loop through tables in PL/SQL?
Look at the following nested loop code example.
DECLARE
CURSOR dept_cur IS
SELECT deptno
FROM dept
ORDER BY deptno;
-- Employee cursor all employees for a dept number
CURSOR emp_cur (v_dept_no DEPT.DEPTNO%TYPE) IS
SELECT ename
FROM emp
WHERE deptno = v_dept_no;
BEGIN
FOR dept_rec IN dept_cur LOOP
dbms_output.put_line('Employees in Department '||TO_CHAR(dept_rec.deptno));
FOR emp_rec in emp_cur(dept_rec.deptno) LOOP
dbms_output.put_line('...Employee is '||emp_rec.ename);
END LOOP;
END LOOP;
END;
/
How often should one COMMIT in a PL/SQL loop? / What is the best commit strategy?
Contrary to popular believe, one should COMMIT less frequently within a PL/SQL loop to prevent ORA-1555 (Snapshot too old) errors. The higher the frequency of commit, the sooner the extents in the rollback segments will be cleared for new transactions, causing ORA-1555 errors.
To fix this problem one can easily rewrite code like this:
FOR records IN my_cursor LOOP
...do some stuff...
COMMIT;
END LOOP;
... to ...
FOR records IN my_cursor LOOP
...do some stuff...
i := i+1;
IF mod(i, 10000) THEN -- Commit every 10000 records
COMMIT;
END IF;
END LOOP;
If you still get ORA-1555 errors, contact your DBA to increase the rollback segments.
NOTE: Although fetching across COMMITs work with Oracle, is not supported by the ANSI standard.
I can SELECT from SQL*Plus but not from PL/SQL. What is wrong?
PL/SQL respect object privileges given directly to the user, but does not observe privileges given through roles. The consequence is that a SQL statement can work in SQL*Plus, but will give an error in PL/SQL. Choose one of the following solutions:
Grant direct access on the tables to your user. Do not use roles!
GRANT select ON scott.emp TO my_user;
Define your procedures with invoker rights (Oracle 8i and higher);
Move all the tables to one user/schema.
What is a mutating and constraining table?
"Mutating" means "changing". A mutating table is a table that is currently being modified by an update, delete, or insert statement. When a trigger tries to reference a table that is in state of flux (being changed), it is considered "mutating" and raises an error since Oracle should not return data that has not yet reached its final state.
Another way this error can occur is if the trigger has statements to change the primary, foreign or unique key columns of the table off which it fires. If you must have triggers on tables that have referential constraints, the workaround is to enforce the referential integrity through triggers as well.
There are several restrictions in Oracle regarding triggers:
A row-level trigger cannot query or modify a mutating table. (Of course, NEW and OLD still can be accessed by the trigger) .
A statement-level trigger cannot query or modify a mutating table if the trigger is fired as the result of a CASCADE delete.
Etc.
Can one pass an object/table as an argument to a remote procedure?
The only way the same object type can be referenced between two databases is via a database link. Note that it is not enough to just use the same type definitions. Look at this example:
-- Database A: receives a PL/SQL table from database B
CREATE OR REPLACE PROCEDURE pcalled(TabX DBMS_SQL.VARCHAR2S) IS
BEGIN
-- do something with TabX from database B
null;
END;
/
-- Database B: sends a PL/SQL table to database A
CREATE OR REPLACE PROCEDURE pcalling IS
TabX DBMS_SQL.VARCHAR2S@DBLINK2;
BEGIN
pcalled@DBLINK2(TabX);
END;
/
Is it better to put code in triggers or procedures? What is the difference?
In earlier releases of Oracle it was better to put as much code as possible in procedures rather than triggers. At that stage procedures executed faster than triggers as triggers had to be re-compiled every time before executed (unless cached). In more recent releases both triggers and procedures are compiled when created (stored p-code) and one can add as much code as one likes in either procedures or triggers.
Is there a PL/SQL Engine in SQL*Plus?
No. Unlike Oracle Forms, SQL*Plus does not have an embedded PL/SQL engine. Thus, all your PL/SQL code is sent directly to the database engine for execution. This makes it much more efficient as SQL statements are not stripped off and sent to the database individually.
Is there a limit on the size of a PL/SQL block?
Yes, the max size is not an explicit byte limit, but related to the parse tree that is created when you compile the code. You can run the following select statement to query the size of an existing package or procedure:
SQL> select * from dba_object_size where name = 'procedure_name';
What are the difference between DDL, DML and DCL commands?
DDL is Data Definition Language statements. Some examples:
CREATE - to create objects in the database
ALTER - alters the structure of the database
DROP - delete objects from the database
TRUNCATE - remove all records from a table, including all spaces allocated for the records are removed
COMMENT - add comments to the data dictionary
GRANT - gives user's access privileges to database
REVOKE - withdraw access privileges given with the GRANT command
DML is Data Manipulation Language statements. Some examples:
SELECT - retrieve data from the a database
INSERT - insert data into a table
UPDATE - updates existing data within a table
DELETE - deletes all records from a table, the space for the records remain
CALL - call a PL/SQL or Java subprogram
EXPLAIN PLAN - explain access path to data
LOCK TABLE - control concurrency
DCL is Data Control Language statements. Some examples:
COMMIT - save work done
SAVEPOINT - identify a point in a transaction to which you can later roll back
ROLLBACK - restore database to original since the last COMMIT
SET TRANSACTION - Change transaction options like what rollback segment to use
How does one escape special characters when building SQL queries?
The LIKE keyword allows for string searches. The '_' wild card character is used to match exactly one character, '%' is used to match zero or more occurrences of any characters. These characters can be escaped in SQL. Example:
SELECT name FROM emp WHERE id LIKE '%\_%' ESCAPE '\';
Use two quotes for every one displayed. Example:
SELECT 'Franks''s Oracle site' FROM DUAL;
SELECT 'A ''quoted'' word.' FROM DUAL;
SELECT 'A ''''double quoted'''' word.' FROM DUAL;
How does one eliminate duplicates rows from a table?
Choose one of the following queries to identify or remove duplicate rows from a table leaving only unique records in the table:
Method 1:
SQL> DELETE FROM table_name A WHERE ROWID > (
2 SELECT min(rowid) FROM table_name B
3 WHERE A.key_values = B.key_values);
Method 2:
SQL> create table table_name2 as select distinct * from table_name1;
SQL> drop table_name1;
SQL> rename table_name2 to table_name1;
SQL> -- Remember to recreate all indexes, constraints, triggers, etc on table...
Method 3:
SQL> delete from my_table t1
SQL> where exists (select 'x' from my_table t2
SQL> where t2.key_value1 = t1.key_value1
SQL> and t2.key_value2 = t1.key_value2
SQL> and t2.rowid > t1.rowid);
Note: One can eliminate N^2 unnecessary operations by creating an index on the joined fields in the inner loop (no need to loop through the entire table on each pass by a record). This will speed-up the deletion process.
Note 2: If you are comparing NOT-NULL columns, use the NVL function. Remember that NULL is not equal to NULL. This should not be a problem as all key columns should be NOT NULL by definition.
How does one generate primary key values for a table?
Create your table with a NOT NULL column (say SEQNO). This column can now be populated with unique values:
SQL> UPDATE table_name SET seqno = ROWNUM;
or use a sequences generator:
SQL> CREATE SEQUENCE sequence_name START WITH 1 INCREMENT BY 1;
SQL> UPDATE table_name SET seqno = sequence_name.NEXTVAL;
Finally, create a unique index on this column.
How does one get the time difference between two date columns
Look at this example query:
select floor(((date1-date2)*24*60*60)/3600)
|| ' HOURS ' ||
floor((((date1-date2)*24*60*60) -
floor(((date1-date2)*24*60*60)/3600)*3600)/60)
|| ' MINUTES ' ||
round((((date1-date2)*24*60*60) -
floor(((date1-date2)*24*60*60)/3600)*3600 -
(floor((((date1-date2)*24*60*60) -
floor(((date1-date2)*24*60*60)/3600)*3600)/60)*60)))
|| ' SECS ' time_difference
from ...
If you don't want to go through the floor and ceiling math, try this method (contributed by Erik Wile):
select to_char(to_date('00:00:00','HH24:MI:SS') +
(date1 - date2), 'HH24:MI:SS') time_difference
from ...
Note that the second query only uses the time-of-day portion of the date fields and thus will never return a value bigger than 23:59:59
How does one add a day/hour/minute/second to a date value?
The SYSDATE pseudo-column shows the current system date and time. Adding 1 to SYSDATE will advance the date by 1 day. Use fractions to add hours, minutes or seconds to the date. Look at these examples:
SQL> select sysdate, sysdate+1/24, sysdate +1/1440, sysdate + 1/86400 from dual;
SYSDATE SYSDATE+1/24 SYSDATE+1/1440 SYSDATE+1/86400
-------------------- -------------------- -------------------- --------------------
03-Jul-2002 08:32:12 03-Jul-2002 09:32:12 03-Jul-2002 08:33:12 03-Jul-2002 08:32:13
The following format is frequently used with Oracle Replication:
select sysdate NOW, sysdate+30/(24*60*60) NOW_PLUS_30_SECS from dual;
NOW NOW_PLUS_30_SECS
-------------------- --------------------
03-JUL-2002 16:47:23 03-JUL-2002 16:47:53
How does one count different data values in a column?
Use this simple query to count the number of data values in a column:
select my_table_column, count(*)
from my_table
group by my_table_column;
A more sophisticated example...
select dept, sum( decode(sex,'M',1,0)) MALE,
sum( decode(sex,'F',1,0)) FEMALE,
count(decode(sex,'M',1,'F',1)) TOTAL
from my_emp_table
group by dept;
How does one count/sum RANGES of data values in a column?
A value x will be between values y and z if GREATEST(x, y) = LEAST(x, z). Look at this example:
select f2,
sum(decode(greatest(f1,59), least(f1,100), 1, 0)) "Range 60-100",
sum(decode(greatest(f1,30), least(f1, 59), 1, 0)) "Range 30-59",
sum(decode(greatest(f1, 0), least(f1, 29), 1, 0)) "Range 00-29"
from my_table
group by f2;
For equal size ranges it might be easier to calculate it with DECODE(TRUNC(value/range), 0, rate_0, 1, rate_1, ...). Eg.
select ename "Name", sal "Salary",
decode( trunc(f2/1000, 0), 0, 0.0,
1, 0.1,
2, 0.2,
3, 0.31) "Tax rate"
from my_table;
Can one retrieve only the Nth row from a table?
Solution#1:
SELECT * FROM (
SELECT ENAME,ROWNUM RN FROM EMP WHERE ROWNUM
Note: Note: In this first it select only one more than the required row, then
it selects the required one. Its far better than using MINUS operation.
Solution#2:
SELECT f1 FROM t1
WHERE rowid = (
SELECT rowid FROM t1
WHERE rownum <= SELECT 10 FROM ROWID T1 10);
Alternatively... SELECT * FROM emp WHERE rownum=1 AND rowid NOT IN
(SELECT rowid FROM emp WHERE rownum
Please note, there is no explicit row order in a relational database. However,
this query is quite fun and may even help in the odd situation.
Can one retrieve only rows X to Y from a table?
SELECT * FROM (
SELECT ENAME,ROWNUM RN FROM EMP WHERE ROWNUM
Note: the 101 is just one greater than the maximum row of the required rows
(means x= 90, y=100, so the inner values is y+1).
Another solution is to use the MINUS operation. For example, to display rows
5 to 7, construct a query like this:
SELECT *
FROM tableX
WHERE rowid in (
SELECT rowid FROM tableX
WHERE rownum <= SELECT TABLEX
Please note, there is no explicit row order in a relational database. However,
this query is quite fun and may even help in the odd situation.
How does one select EVERY Nth row from a table?
One can easily select all even, odd, or Nth rows from a table using SQL queries like this:
Solution 1: Using a subquery
SELECT *
FROM emp
WHERE (ROWID,0) IN (SELECT ROWID, MOD(ROWNUM,4)
FROM emp);
Solution 2: Use dynamic views (available from Oracle7.2):
SELECT *
FROM ( SELECT rownum rn, empno, ename
FROM emp
) temp
WHERE MOD(temp.ROWNUM,4) = 0;
Please note, there is no explicit row order in a relational database. However,
these queries are quite fun and may even help in the odd situation.
How does one select the TOP N rows from a table?
Form Oracle8i one can have an inner-query with an ORDER BY clause.
Look at this example:
SELECT *
FROM (SELECT * FROM my_table ORDER BY col_name_1 DESC)
WHERE ROWNUM
Use this workaround with prior releases:
SELECT *
FROM my_table a
WHERE 10 >= (SELECT COUNT(DISTINCT maxcol)
FROM my_table b
WHERE b.maxcol >= a.maxcol)
ORDER BY maxcol DESC;
How does one code a tree-structured query?
Tree-structured queries are definitely non-relational (enough to kill Codd and
make him roll in his grave). Also, this feature is not often found in other
database offerings.
The SCOTT/TIGER database schema contains a table EMP with a self-referencing
relation (EMPNO and MGR columns). This table is perfect for tesing and
demonstrating tree-structured queries as the MGR column contains the employee
number of the "current" employee's boss.
The LEVEL pseudo-column is an indication of how deep in the tree one is.
Oracle can handle queries with a depth of up to 255 levels. Look at this
example:
select LEVEL, EMPNO, ENAME, MGR
from EMP
connect by prior EMPNO = MGR
start with MGR is NULL;
One can produce an indented report by using the level number to substring or
lpad() a series of spaces, and concatenate that to the string. Look at this
example:
select lpad(' ', LEVEL * 2) || ENAME ........
One uses the "start with" clause to specify the start of the tree. More than
one record can match the starting condition. One disadvantage of having a
"connect by prior" clause is that you cannot perform a join to other tables.
The "connect by prior" clause is rarely implemented in the other database
offerings. Trying to do this programmatically is difficult as one has to do
the top level query first, then, for each of the records open a cursor to look
for child nodes.
One way of working around this is to use PL/SQL, open the driving cursor with
the "connect by prior" statement, and the select matching records from other
tables on a row-by-row basis, inserting the results into a temporary table
for later retrieval.
How does one code a matrix report in SQL?
Look at this example query with sample output:
SELECT *
FROM (SELECT job,
sum(decode(deptno,10,sal)) DEPT10,
sum(decode(deptno,20,sal)) DEPT20,
sum(decode(deptno,30,sal)) DEPT30,
sum(decode(deptno,40,sal)) DEPT40
FROM scott.emp
GROUP BY job)
ORDER BY 1;
JOB DEPT10 DEPT20 DEPT30 DEPT40
--------- ---------- ---------- ---------- ----------
ANALYST 6000
CLERK 1300 1900 950
MANAGER 2450 2975 2850
PRESIDENT 5000
SALESMAN 5600
How does one implement IF-THEN-ELSE in a select statement?
The Oracle decode function acts like a procedural statement inside an
SQL statement to return different values or columns based on the values of
other columns in the select statement.
Some examples:
select decode(sex, 'M', 'Male', 'F', 'Female', 'Unknown')
from employees;
select a, b, decode( abs(a-b), a-b, 'a > b',
0, 'a = b',
'a
Note: The decode function is not ANSI SQL and is rarely implemented
in other RDBMS offerings. It is one of the good things about Oracle,
but use it sparingly if portability is required.
From Oracle 8i one can also use CASE statements in SQL. Look at this example:
SELECT ename, CASE WHEN sal>1000 THEN 'Over paid' ELSE 'Under paid' END
FROM emp;
How can one dump/ examine the exact content of a database column?
SELECT DUMP(col1)
FROM tab1
WHERE cond1 = val1;
DUMP(COL1)
-------------------------------------
Typ=96 Len=4: 65,66,67,32
For this example the type is 96, indicating CHAR, and the last byte in the column is 32, which is the ASCII code for a space.
This tells us that this column is blank-padded.
Can one drop a column from a table?
From Oracle8i one can DROP a column from a table.
1. SQL> update t1 set column_to_drop = NULL;
SQL> rename t1 to t1_base;
SQL> create view t1 as select
2. SQL> create table t2 as select
SQL> drop table t1;
SQL> rename t2 to t1;
Can one rename a column in a table?
No, this is listed as Enhancement Request 163519. Some workarounds:
1. -- Use a view with correct column names...
rename t1 to t1_base;
create view t1
2. -- Recreate the table with correct column names...
create table t2
drop table t1;
rename t2 to t1;
3. -- Add a column with a new name and drop an old column...
alter table t1 add ( newcolame datatype );
update t1 set newcolname=oldcolname;
alter table t1 drop column oldcolname;
How can I change my Oracle password?
Issue the following SQL command:
ALTER USER
/
From Oracle8 you can just type "password" from SQL*Plus,
or if you need to change another user's password, type "password user_name".
How does one find the next value of a sequence?
Perform an "ALTER SEQUENCE ... NOCACHE" to unload the unused cached sequence
numbers from the Oracle library cache. This way, no cached numbers will be lost.
If you then select from the USER_SEQUENCES dictionary view, you will
see the correct high water mark value that would be returned for the next
NEXTVALL call. Afterwards, perform an "ALTER SEQUENCE ... CACHE" to restore
caching.
You can use the above technique to prevent sequence number loss before
a SHUTDOWN ABORT, or any other operation that would cause gaps in
sequence values.
Workaround for snapshots on tables with LONG columns
You can use the SQL*Plus COPY command instead of snapshots if you need to copy LONG and LONG RAW variables from one location to another. Eg:
COPY TO SCOTT/TIGER@REMOTE -
CREATE IMAGE_TABLE USING -
SELECT IMAGE_NO, IMAGE -
FROM IMAGES;
Note: If you run Oracle8, convert your LONGs to LOBs, as it can be replicated.
posted by Nagz @ 11:15 PM
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