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What are joins..how many types of joins are there?
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Answer Posted / sam s. kolta
Joining Types:
1. Joining according to Condition Existence Join:
a. Cross or Cartesian Join: SELECT * FROM emp
CROSS JOIN dept OR SELECT * FROM emp, dept;
b. Restriction Join: SELECT * FROM emp;
c. Natural Join: SELECT d.dname,
d.loc, e.ename, e.job FROM dept d NATURAL JOIN emp e;
d. Named Column Join: SELECT empno, ename,
dname FROM emp natural join dept;
e. Conditional Join: SELECT e.empno,
e.ename, e.job, d.dname, d.loc FROM emp e, dept d Where
e.deptno = d.deptno;
2. Joining according to Row Selection Join:
a. Inner Join: SELECT d.dname, d.loc,
e.ename, e.job FROM dept d INNER JOIN emp e USING (deptno);
b. Left Outer: SELECT deptno, d.dname,
d.loc, e.ename, e.job FROM dept d LEFT OUTER JOIN emp e
USING (deptno);
c. Right Outer: SELECT deptno, d.dname,
d.loc, e.ename, e.job FROM emp e RIGHT OUTER JOIN dept d
USING (deptno);
d. Full Outer: SELECT d.dname, e.ename
FROM dept d LEFT OUTER JOIN emp e ON d.deptno = e.deptno;
3. Joining according to comparison operator:
a. Equijoin: SELECT e.empno,
e.ename,e.sal,d.deptno,d.dname,d.loc FROM emp e, dept d
WHERE e.deptno = d.deptno;
b. Non Equijoin: SELECT e.empno, e.ename,
e.sal, s.grade FROM emp e, salgrade s WHERE e.sal between
s.losal and s.hisal;
c. Predict Operator: SELECT d.dname, d.loc,
e.ename, e.job FROM dept d LEFT OUTER JOIN emp e ON
(d.deptno = e.deptno); WHERE
d.loc = 'DALLAS';
4. Joining according to used table (Self Join):
a. Inner Join: SELECT b. ename, a.
mgr from emp a left outer join emp b on (a.empno = b.mgr);
b. Outer Join: SELECT b. ename, a.
mgr from emp a right outer join emp b on (a.empno = b.mgr);
5. Joining according to ANTI or SEMI JOIN:
a. ANTI JOIN: SELECT D.deptno, D.dname
FROM dept D, emp E WHERE E.deptno = D.deptno ORDER BY
D.deptno;
b. SEMI JOIN: SELECT D.deptno, D.dname
FROM dept D WHERE EXISTS (SELECT 1 FROM emp E WHERE
E.deptno = D.deptno) ORDER BY D.deptno;
________________________________________
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