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TABLE OF CONTENTS
Overview
How can I transform a subquery involving the IN clause to a Join?
How can I transform a statement involving an OR condition to a UNION ALL?
How do I eliminate duplicate values in a table?
How can I get a count of the different data values in a column?
How can I get count/sum RANGES of data values in a column?
How can I get the time difference between two date columns?
Can I retrieve only the Nth row from a table?
Can I retrieve only rows X to Y from a table?
Can I retrieve every Nth row from a table?
Retrieve the TOP N Rows from a table?
Is it possible to dump/examine the exact content of a database column?
Can I code a MATRIX Report using SQL?
Tree-structured (hierarchical) queries
Overview
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Structured Query Language (SQL) is an implementation of a "data sublanguage" used in almost all relational database systems. SQL was developed by IBM in the early 1970s for use in System R, and is "de facto" standard, as well as an ISO and ANSI standard.
In common usage, SQL supports four types of SQL statements:
Queries: This type of SQL statement implements the standard relational operations such as SELECTION, PROJECTION and JOIN. The SELECT statement forms the basis for many other SQL operations. You can use SELECT statements to query tables, create views, in subqueries, or when creating a table defined as the result set of query.
Data Manipulation Language (DML): DML statements are used to allow data in the database to be added, amended or deleted. DML functionality is provided by the use of INSERT, UPDATE and DELETE statements.
Data Definition Language (DDL): This is the language that allows for objects to be created or altered. Provided below are a few example Oracle DDL statements:
Data Control Language (DCL): This is the language that allows for transaction control. Provided below are a few example Oracle DCL statements:
The development of SQL is governed by standards. A major revision of the SQL standard was completed in 1992, called SQL2. SQL3 is a newer revision that supports object extensions and will be partially implemented starting with Oracle8.
How can I transform a subquery involving the IN clause to a Join?
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Original statement:
SELECT e.empno, e.ename
FROM emp e
WHERE e.deptno IN (
SELECT d.deptno
FROM dept d
WHERE d.loc = 'CHICAGO'
);
New statement: SELECT e.empno, e.ename FROM emp e, dept d WHERE d.loc = 'CHICAGO' AND d.deptno = e.deptno;
How can I transform a statement involving an OR condition to a UNION ALL?
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Original statement:
SELECT dname, loc
FROM dept
WHERE loc = 'CHICAGO'
OR loc = 'NEW YORK';
New Statement:
SELECT dname, loc FROM dept WHERE loc = 'CHICAGO' UNION ALL SELECT dname, loc FROM dept WHERE loc = 'NEW YORK';
How do I eliminate duplicate values in a table?
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Provided below are four methods for identifying or removing duplicate rows from a table:
Method 1:
DELETE FROM emp a
WHERE rowid > (
SELECT min(rowid)
FROM emp b
WHERE a.emp_id = b.emp_id
);
Method 2:
CREATE TABLE emp2 AS
SELECT distinct *
FROM emp;
DROP TABLE emp;
RENAME emp2 TO emp;
Method 3:
DELETE FROM emp
WHERE rowid NOT IN (
SELECT MIN(rowid)
FROM emp
GROUP BY emp_id
);
Method 4:
DELETE FROM emp a
WHERE EXISTS (
SELECT 'true'
FROM emp b
WHERE b.emp_id = a.emp_id
AND b.rowid < a.rowid
);
NOTE: If you where to create an index on the joined fields in the inner loop, it may be possible to eliminate N^2 operations as there would be no need to loop through the entire table on each pass be a record.
How can I get a count of the different data values in a column?
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SELECT
dname
, sum(decode(job, 'CLERK', 1, 0)) Clerk
, sum(decode(job, 'SALESMAN', 1, 0)) Salesman
, sum(decode(job, 'MANAGER', 1, 0)) Manager
, sum(decode(job, 'ANALYST', 1, 0)) Analyst
, sum(decode(job, 'PRESIDENT', 1, 0)) President
FROM
emp e
, dept d
WHERE
e.deptno (+) = d.deptno
GROUP BY
dname;
DNAME CLERK SALESMAN MANAGER ANALYST PRESIDENT
-------------- ---------- ---------- ---------- ---------- ----------
ACCOUNTING 1 0 1 0 1
OPERATIONS 0 0 0 0 0
RESEARCH 2 0 1 2 0
SALES 1 4 1 0 0
How can I get count/sum RANGES of data values in a column?
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A value "x" will be between values "y" and "z" if: GREATEST(x,y)=LEAST(x,z).
SELECT
job
, sum(decode(greatest(sal,2999), least(sal,6000), 1, 0)) "Range 3000-6000"
, sum(decode(greatest(sal,1000), least(sal,2999), 1, 0)) "Range 1000-3000"
, sum(decode(greatest(sal,0), least(sal,999), 1, 0)) "Range 0-1000"
FROM emp
GROUP BY job;
JOB Range 3000-6000 Range 1000-3000 Range 0-1000
--------- --------------- --------------- ------------
ANALYST 2 0 0
CLERK 0 2 2
MANAGER 0 3 0
PRESIDENT 1 0 0
SALESMAN 0 4 0
How can I get the time difference between two date columns?
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Method 1: (Very long-winded. A more efficient version is indicated in Method 2.)
SELECT floor(((e1.hiredate-e2.hiredate)*24*60*60)/3600)
|| ' HOURS ' ||
floor((( (e1.hiredate-e2.hiredate)*24*60*60) -
floor(((e1.hiredate-e2.hiredate)*24*60*60)/3600)*3600)/60)
|| ' MINUTES ' ||
round((( (e1.hiredate-e2.hiredate)*24*60*60) -
floor(( (e1.hiredate-e2.hiredate)*24*60*60)/3600)*3600 -
(floor((((e1.hiredate-e2.hiredate)*24*60*60) -
floor(((e1.hiredate-e2.hiredate)*24*60*60)/3600)*3600)/60)*60))
)
|| ' SECONDS ' time_difference
FROM emp e1, emp e2
WHERE e1.ename = 'BLAKE'
AND e2.ename = 'CLARK'
TIME_DIFFERENCE
-------------------------------
162 HOURS 44 MINUTES 25 SECONDS
Method 2: (Thanks to Chris Hunt, http://www.extracon.com/, for providing me with this much easier and efficient SQL query to produce the same results!)
SELECT floor((e1.hiredate-e2.hiredate)*24)
|| ' HOURS ' ||
mod(floor((e1.hiredate-e2.hiredate)*24*60),60)
|| ' MINUTES ' ||
mod(floor((e1.hiredate-e2.hiredate)*24*60*60),60)
|| ' SECONDS ' time_difference
FROM emp e1, emp e2
WHERE e1.ename = 'BLAKE'
AND e2.ename = 'CLARK'
TIME_DIFFERENCE
-------------------------------
162 HOURS 44 MINUTES 25 SECONDS
Can I retrieve only the Nth row from a table?
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Method 1:
SELECT
ename
, job
, hiredate
FROM
emp
WHERE
rowid = (
SELECT rowid FROM emp
WHERE rownum <= 3
MINUS
SELECT rowid FROM emp
WHERE rownum < 3
);
Method 2:
SELECT
ename
, job
, hiredate
FROM
emp
WHERE
rownum = 1
AND rowid NOT IN (
SELECT rowid FROM emp
WHERE rownum < 3
);
NOTE: Always remember that there is no explicit order in a relational database.
Can I retrieve only rows X to Y from a table?
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SELECT
ename
, job
, hiredate
FROM
emp
WHERE
rowid in (
SELECT rowid FROM emp
WHERE rownum <= 7
MINUS
SELECT rowid FROM emp
WHERE rownum < 3
);
Can I retrieve EVERY Nth row from a table?
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Method 1:
SELECT
ename
, job
, hiredate
FROM
emp
WHERE
(rowid,0) in (
SELECT rowid, mod(rownum,4)
FROM emp
);
Method 2: Using Dynamic Views: (available in Oracle7.2 and higher)
SELECT
ename
, job
, hiredate
FROM ( SELECT rownum rn, empno, ename, job, hiredate
FROM emp
) d_table
WHERE mod(d_table.rn,4) = 0;
Retrieve the TOP N Rows from a table?
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Method 1: Starting with Oracle8i, you can have an inner-query with an ORDER BY clause
SELECT
ename
, job
, hiredate
, sal
FROM ( SELECT empno, ename, job, hiredate, sal
FROM emp
ORDER BY sal DESC
)
WHERE rownum < 6;
Method 2: The following workaround will work with prior releases.
SELECT
ename
, job
, hiredate
, sal
FROM
emp a
WHERE 5 >= ( SELECT count(distinct b.sal)
FROM emp b
WHERE b.sal >= a.sal
)
ORDER BY a.sal DESC
Is it possible to dump/examine the exact content of a database column?
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SELECT ename, job, dump(job) FROM emp WHERE ename = 'SMITH'; ENAME JOB DUMP(JOB) ---------- --------- --------------------------- SMITH CLERK Typ=1 Len=5: 67,76,69,82,75
| Type Code | Oracle Datatype |
Oracle type codes for column dumps
Can I code a MATRIX Report using SQL?
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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 emp e
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
Tree-structured (hierarchical) queries
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SELECT lpad(' ', level*4) || ename
FROM emp
CONNECT BY PRIOR empno = mgr
START WITH mgr IS NULL;
Employee Name
------------------------
KING
JONES
JHUNTER
ADAMS
FORD
SMITH
BLAKE
ALLEN
WARD
MARTIN
TURNER
JAMES
CLARK
MILLER
NOTE: Using the "EMP" table from the SCOTT/TIGER schema makes a great test table since it has a "self-referencing" relation. (The MGR column contains the employee number of the "current" employee's boss.
The LEVEL pseudo-column is an indication of hwo deep in the tree you are. Oracle can handle queries with a depth up to 255 levels.
The "START WITH" clause is used 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.
This was first published in June 2004
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