select column only if it exists

i need to display tablespace attributes SQL> select tablespace_name, status, contents, logging, extent_management, allocation_type, segment_space_management, retention, bigfile from dba_tablespaces; TABLESPACE_NAME STATUS CONTENTS LOGGING EXTENT_MAN ALLOCATIO SEGMEN RETENTION BIG ——————– ——— ——— ——— ———- ——— —— ———– — SYSTEM ONLINE PERMANENT LOGGING LOCAL SYSTEM MANUAL NOT APPLY NO looks fine. but what if I try that […]

dynamic number of columns

probably one of the most frequently asked question, you want to have a table like             ACCOUNTING OPERATIONS RESEARCH SALES ANALYST              0          0        2     0 CLERK                1          0        2     1 MANAGER              0          0        1     1 PRESIDENT            1          0        0     0 SALESMAN             0          0        0     4 but the number and name of columns must be dynamic. typically, I answer : this is not possible in plain sql. you need to use plsql. well. It is not too much beautifoul, but […]

pivot table

big mission today : explain the mess with pivot table. let’s look the following query SQL> select extract(year from hiredate) year, count(*) from emp group by extract(year from hiredate) order by year YEAR COU —– — 1980   1 1981  10 1982   1 1987   2 how do we get the years without hiredate with 0? 1) […]

group by does not sort

An user just posted an interresting question today. Why the Group By is crazy? I summarize his example SQL> SELECT NUM FROM (select 400 num from dual union select 310 from dual union select 220000 from dual) group by num,null;     NUM ——-     400  220000     310 Well, group by is “sorting”, but how? this seems crazy. […]