1. Can one retrieve only rows X to Y from a table?
SELECT * FROM (
SELECT ename, rownum rn
FROM emp WHERE rownum < 101
) WHERE RN between 91 and 100 ;
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).
SELECT rownum, f1 FROM t1
GROUP BY rownum, f1 HAVING rownum BETWEEN 2 AND 4;
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 <= 7
MINUS
SELECT rowid FROM tableX
WHERE rownum < 5);
"this one was faster for me and allowed for sorting before filtering by rownum. The inner query (table A) can be a series of tables joined together with any operation before the filtering by rownum is applied."
SELECT *
FROM (SELECT a.*, rownum RN
FROM (SELECT *
FROM t1 ORDER BY key_column) a
WHERE rownum <=7)
WHERE rn >=5;
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.
The generic solution to get full information of rows between x and y
SELECT * FROM emp WHERE empno in (SELECT empno FROM emp GROUP BY rownum,empno HAVING rownum BETWEEN &x AND &y);
"select particular rows from a table : select for rownum = 4, 15 and 17."
select * from (
select rownum myrownum, emp.* from employees emp
) mytable
where myrownum in (4,15,17);
"selecting row between range of rownum: select for rownum between (12, 20)."
select * from (
select rownum myrownum, emp.* from employees emp
) mytable
where myrownum between 12 and 20;
"Replace 12 and 20 with &x and &y respectively to assign range dynamically."
select * from (
select rownum myrownum, emp.* from employees emp
) mytable
where myrownum between &x and &y;
"Combined query to give complete flexibility to pick particular rows and also a given range."
select * from (
select rownum myrownum, emp.* from employees emp
) mytable
where myrownum between 12 and 17
or myrownum in ( 3, 18, 25);
2. Can one retrieve only the Nth row from a table?
SELECT * FROM t1 a
WHERE n = (SELECT COUNT(rowid)
FROM t1 b
WHERE a.rowid >= b.rowid);
SELECT * FROM (
SELECT ENAME,ROWNUM RN FROM EMP WHERE ROWNUM < 101 )
WHERE RN = 100;
Note: In this first query we select one more than the required row number, then we select the required one. Its far better than using a MINUS operation.
SELECT f1 FROM t1
WHERE rowid = (
SELECT rowid FROM t1
WHERE rownum <= 10
MINUS
SELECT rowid FROM t1
WHERE rownum < 10);
SELECT rownum,empno FROM scott.emp a
GROUP BY rownum,empno HAVING rownum = 4;
Alternatively...
SELECT * FROM emp WHERE rownum=1 AND rowid NOT IN
(SELECT rowid FROM emp WHERE rownum < 10);
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.
3. How can one dump/ examine the exact content of a database column?
Table data can be extracted from the database as octal, decimal or hex values:
SELECT DUMP(col1, 10)
FROM tab1
WHERE cond1 = val1;
DUMP(COL1)
-------------------------------------
Typ=96 Len=4: 65,66,67,32
For this example, type=96 is indicating a CHAR column. 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.
4. How does one add a column to the middle of a table?
Oracle only allows columns to be added to the end of an existing table. Example:
SQL> CREATE TABLE tab1 ( col1 NUMBER );
Table created.
SQL> ALTER TABLE tab1 ADD (col2 DATE);
Table altered.
SQL> DESC tab1
Name Null? Type
----------------------------------------- -------- --------------------COL1 NUMBER
COL2 DATE
Nevertheless, some databases also allow columns to be added to an existing table after a particular column (i.e. in the middle of the table). For example, in MySQL the following syntax is valid:
ALTER TABLE tablename ADD columnname AFTER columnname;
Oracle does not support this syntax. However, it doesn't mean that it cannot be done.
Workarounds:
1. Create a new table and copy the data across.
SQL> RENAME tab1 TO tab1_old;
Table renamed.
SQL> CREATE TABLE tab1 AS SELECT 0 AS col1, col1 AS col2 FROM tab1_old;
Table created.
2. Use the DBMS_REDEFINITION package to change the structure on-line while users are working.
5. How does one code a hierarchical tree-structured query?
The SCOTT/TIGER database schema contains a table EMP with a self-referencing relation (EMPNO and MGR columns). This table is perfect for testing 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:
SQL> SELECT level, empno, ename, mgr
2 FROM emp
3 CONNECT BY PRIOR empno = mgr
4 START WITH mgr IS NULL
5 /
LEVEL EMPNO ENAME MGR
---------- ---------- ---------- ----------
1 7839 KING
2 7566 JONES 7839
3 7788 SCOTT 7566
...
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:
SQL> SELECT LPAD(' ', LEVEL * 2) || ename
2 FROM emp
3 CONNECT BY PRIOR empno = mgr
4 START WITH mgr IS NULL;
LPAD(,LEVEL*2)||ENAME
------------------------------------------------------
KING
JONES
SCOTT
...
Use 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.
NOTE: 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.
6. How does one count/sum data values in a column?
Count/sum FIX values:
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;
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;
7. How does one drop/ rename a column in a table?
Drop a column
From Oracle 8i one can DROP a column from a table. Look at this sample script, demonstrating the ALTER TABLE table_name DROP COLUMN column_name; command.
Workarounds for older releases:
SQL> update t1 set column_to_drop = NULL;
SQL> rename t1 to t1_base;
SQL> create view t1 as select >specific columns> from t1_base;
SQL> create table t2 as select >specific columns> from t1;
SQL> drop table t1;
SQL> rename t2 to t1;
Rename a column
From Oracle 9i one can RENAME a column from a table. Look at this example:
ALTER TABLE tablename RENAME COLUMN oldcolumn TO newcolumn;
Workarounds for older releases:
Use a view with correct column names:
rename t1 to t1_base;
create view t1 >column list with new name> as select * from t1_base;
Recreate the table with correct column names:
create table t2 >column list with new name> as select * from t1;
drop table t1;
rename t2 to t1;
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;
8. How does one implement IF-THEN-ELSE logic in a SELECT statement?
One can use the CASE statement or functions like DECODE, NVL, NVL2, NULLIF, COALESCE, etc.
Here is the syntax for the CASE-statement:
CASE exp WHEN comparison_exp1 THEN return_exp1
[WHEN comparison_exp2 THEN return_exp2
WHEN comparison_exp3 THEN return_exp3
ELSE else_exp
]
END
And for DECODE:
DECODE( col | exprn, srch1, rslt1 [, srch2, rslt2,...,] [,default] )
9. How does one prevent Oracle from using an Index?
In certain cases, one may want to disable the use of a specific, or all indexes for a given query. Here are some examples:
Adding an expression to the indexed column
SQL>select count(*) from t where empno+0=1000;
COUNT(*)
----------
1
Execution Plan
--------------------------------------------- ----- --------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=3)
1 0 SORT (AGGREGATE)
2 1 TABLE ACCESS (FULL) OF 'T' (Cost=2 Card=1 Bytes=3)
Specifying the FULL hint to force full table scan
SQL>select /*+ FULL(t) */ * from t where empno=1000;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO GRADE
---------- ---------- --------- ---------- --------- ---------- ------- 1000 Victor DBA 7839 20-MAY-03 11000 0 10 JUNIOR
Execution Plan
--------------------------------------------- ----- --------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=41)
1 0 TABLE ACCESS (FULL) OF 'T' (Cost=2 Card=1 Bytes=41)
Specifying NO_INDEX hint
SQL>select /*+ NO_INDEX(T) */ count(*) from t where empno=1000;
COUNT(*)
----------
1
Execution Plan
--------------------------------------------- ----- --------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=3)
1 0 SORT (AGGREGATE)
2 1 TABLE ACCESS (FULL) OF 'T' (Cost=2 Card=1 Bytes=3)
Using a function over the indexed column
SQL>select count(*) from t where to_number(empno)=1000;
COUNT(*)
----------
1
Execution Plan
--------------------------------------------- ----- --------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=3)
1 0 SORT (AGGREGATE)
2 1 TABLE ACCESS (FULL) OF 'T' (Cost=2 Card=1 Bytes=3)
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