sql faq

7/31/2019 SQL FAQ

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Document ID: 2194.1

Subject: COMMON SQL*PLUS QUESTIONS AND ANSWERS

Author: ILAHIRI

Q: What is the purpose of the COPY command ?

A: Copy command in SQL*Plus is useful when you want to transfer data between Oracle and NON-Oracle databases. If you wantto copy Oracle table to another Oracle table, use"create table .. as select .." or "create table, insert into.. select .." statements.Columns may lose precision if you copy from Oraclerdbms to Oracle rdbms.

Q: How can I select data to the 'same line' in SQL*Plus:

A: For example you want to show sum_of_salaries/department inone line instead of 4 lines:

select sum(decode(deptno,10,sal,0)) sal_10,sum(decode(deptno,20,sal,0)) sal_20,sum(decode(deptno,30,sal,0)) sal_30,sum(decode(deptno,40,sal,0)) sal_40

from emp;

Q: How to start a SQL*Plus script when at least one row is foundfrom a table ? If no rows are found, then nothing should bestarted.

A: Use a script like the following:spool tmp.sqlSelect decode(greatest(count(*),0),'0','start dummy','start script')from tablex where colx = ¶meter;spool off

@tmp

It will start the dummy (which does nothing) if no rows arereturned, otherwize it will start the "script".

Q: How to use "set pause" in SQL*Plus ?

A: For example to get 'Underlined prompt' in a VT220terminal use the following code in your login.sql:

set pause [1m[4m'Underline prompt'[mset pause on

Note that the must be a real esc, not the characters <,E,S,Cand >.

Q: How to show all of a long field ?

A: set long 4000set array 1

Q: I get "ORA-907: Missing right parenthesis", but I have enoughparenthesis ? My select statement is:

select * from empwhere ('A%' is null

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or empno in (select empno from emp e emp suwhere empno=7999))and 1=1;

A: Sometimes the error messages are misleading,the actual error is a missing comma on line 3.

Q: Why is my update script getting slow when I update a lot of rows ?

A: If you want to update many rows in a table by a PL/SQL block,and you use a cursor to that table, the rollback segments arereleased only when you CLOSE the cursor. Thus, as time passes withthe cursor still open, the process slows down because it has tokeep searching an increasingly large rollback segment to build theconsistent picture of the the table as it was when the cursor wasopened. To avoid this problem, try to use a flag in the tabledescribing what rows are not updated and then close + re-open thecursor every 1000 rows or so.

Q: Why I don't get the "No rows selected" if I issueselect max(empno) from emp where deptno = -10 ?There are no emps with deptno -10.

A: The GROUP functions (min/max/avg...)will ALWAYS fetch at least one row unless youspecify a group by clause.

Q: How can I remove duplicates from my table ?

A: To find duplicate keys from a table tx:select key, count(key) no_of_duplicatesfrom txgroup by keyhaving count(key) > 1;

To remove duplicates from table txwhen IT DOES NOT MATTER WHICH ONE IS SAVED:

delete from txwhere key not in

(select min(key) from tx group by key);

Of course you can choose which one to save in the subselect.

Q: How to get the Operating system userid ?

A: select s.usernamefrom v$session s, v$process pwhere s.paddr = p.addr and s.username = user

This works when you don't use SQL*Net connections.

Q: How to find the SID of the database in Forms/Plus etc ?

A: "Select value from sys.v$parameter where name ='db_name';"Remember to grant select on that table to public usinguser SYS.

Q: Ttitle do not show up if no rows are selected in a select statement. Howto get around that.

A: You have to select something that will definitely succed and then do a

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noprint on that. Below is the relevent workaround.ttitle center 'testing'column x noprintselect 'x' x from dual;select * from dept where deptno=77;/the result :

testing

no rows selected

Q: How to supress the date and page nos. from appearing .

A: By default the date and the time will appear if you just do a ttitle anddo not qualify it. Eg.SQL> ttitle 'test'SQL> select * from dual;Output:September 3, 1992 page 1If you do not want date and page no.qualify the ttitle with something(any of the clauses mentioned in pp 4-18 of sqlplus ref manual). eg.

SQL> ttitle left 'test'SQL> select * from dual;

testD-X

Q: How to show the time difference between two date columns ?

A: 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 ' timediff from ...

Q: For Add_months() function if the first parameter is the last day of aa month (i.e. '28-feb-91'), the result will be the last day of themonth (i.e. '31-aug-91'); the 27th of feb, of course, returns the27th of aug. Is there any way to override this rule so thatadd_months('28-feb-91',6) will return '28-AUG-91'?

A: select to_date(add_months(c1-1,6))+1 alias_col_name from Table;where c1 is a date col holding the value 28-FEB-91.

Q: I tried to use UNION but I gotORA-01789: query block has incorrect number of result columns.I have only some of the columns in the other table.

A: You have to define the missing columns. Note the number of spaces between the single quotes.

- character:select ' ', ename from foo -- One space between the single quotesunion

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select charcol, ename from table;

- number:select 0, ename from foounionselect numericcol, ename from table

- date:select to_date(''), ename from foo --No space between the single quotesunionselect datecol, ename from table

Q: How do you find the days between two dates excluding weekends (Saturdaysand Sundays)?

A: This is a SQL SELECT statement that subtracts two dates and givesyou the number of days between them, excluding weekends.

SELECT (TRUNC(enddate,'D') - TRUNC(begdate+6,'D'))- (((TRUNC(enddate,'D') - TRUNC(begdate+6,'D'))/7)*2)+ DECODE(TO_NUMBER(TO_CHAR(enddate,'D')),3,1,4,2,5,3,6,4,7,5,0)+ LEAST(7-DECODE)(TO_NUMBER(TO_CHAR(begdate,'D')),1,7,

TO_NUMBER(TO_CHAR(begdate,'D'))),5) DAYSFROM datetable

Q. I have a number field which holds time in seconds. How can I displaythis using an oracle time format?

A. select to_char(trunc(sysdate)+(time_field/86400),'HH:MI:SS')from time_table;

*86400 is the number of seconds in a day

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