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MySQL non-equi join

When we are joining the tables, we write the condition in ON clause using many operators. If we use any other operator other than “=” operator like (>, <, AND, OR, BETWEEN) then we call it as non-equi join.

Below is an example for non-equi join to EMP table with DEPT table.

SELECT E.*, D.*

FROM DEPT D JOIN EMP E

ON D.DEPTNO != E.DEPTNO;

mysql-non-equi-join-0
 

CREATE TABLE DEPT

       (DEPTNO NUMERIC(2) ,

        DNAME VARCHAR(14) ,

        LOC VARCHAR(13),

CONSTRAINT PK_DEPT PRIMARY KEY(DEPTNO)

 ) ;

 

CREATE TABLE EMP

       (EMPNO NUMERIC(4) ,

        ENAME VARCHAR(10),

        JOB VARCHAR(9),

        MGR NUMERIC(4),

        HIREDATE DATE,

        SAL NUMERIC(7,2),

        COMM NUMERIC(7,2),

        DEPTNO NUMERIC(2),

 CONSTRAINT FK_DEPTNO FOREIGN KEY(DEPTNO) REFERENCES DEPT(DEPTNO),

CONSTRAINT PK_EMP PRIMARY KEY(EMPNO)

 

);

INSERT INTO DEPT VALUES (10,'ACCOUNTING','NEW YORK');

INSERT INTO DEPT VALUES (20,'RESEARCH','DALLAS');

INSERT INTO DEPT VALUES (30,'SALES','CHICAGO');

INSERT INTO DEPT VALUES (40,'OPERATIONS','BOSTON');

 

INSERT INTO EMP VALUES (7369,'SMITH','CLERK',7902,'1980-12-17',800,NULL,20);

INSERT INTO EMP VALUES (7499,'ALLEN','SALESMAN',7698,'1981-2-20',1600,300,30);

INSERT INTO EMP VALUES (7521,'WARD','SALESMAN',7698,'1981-2-22',1250,500,30);

INSERT INTO EMP VALUES (7566,'JONES','MANAGER',7839,'1981-4-2',2975,NULL,20);

INSERT INTO EMP VALUES (7654,'MARTIN','SALESMAN',7698,'1981-9-28',1250,1400,30);

INSERT INTO EMP VALUES (7698,'BLAKE','MANAGER',7839,'1981-5-1',2850,NULL,30);

INSERT INTO EMP VALUES (7782,'CLARK','MANAGER',7839,'1981-6-9',2450,NULL,10);

INSERT INTO EMP VALUES (7788,'SCOTT','ANALYST',7566,'1987-4-19',3000,NULL,20);

INSERT INTO EMP VALUES (7839,'KING','PRESIDENT',NULL,'1981-11-17',5000,NULL,10);

INSERT INTO EMP VALUES (7844,'TURNER','SALESMAN',7698,'1981-9-8',1500,0,30);

INSERT INTO EMP VALUES (7876,'ADAMS','CLERK',7788,'1987-5-23',1100,NULL,20);

INSERT INTO EMP VALUES (7900,'JAMES','CLERK',7698,'1981-12-3',950,NULL,30);

INSERT INTO EMP VALUES (7902,'FORD','ANALYST',7566,'1981-12-3',3000,NULL,20);

INSERT INTO EMP VALUES (7934,'MILLER','CLERK',7782,'1982-1-23',1300,NULL,10);

COMMIT;

mysql-non-equi-join-1
 

Non-equi join example for joining two tables which are Non Related

CREATE TABLE SALGRADE

      ( GRADE NUMERIC,

        LOSAL NUMERIC,

        HISAL NUMERIC );

 

INSERT INTO SALGRADE VALUES (1,700,1200);

INSERT INTO SALGRADE VALUES (2,1201,1400);

INSERT INTO SALGRADE VALUES (3,1401,2000);

INSERT INTO SALGRADE VALUES (4,2001,3000);

INSERT INTO SALGRADE VALUES (5,3001,9999);

COMMIT;

mysql-non-equi-join-2

SELECT E.ENAME, E.SAL, S.GRADE

FROM EMP E JOIN SALGRADE S

ON E.SAL >= S.LOSAL AND E.SAL <= S.HISAL;

mysql-non-equi-join-3

CARTESIAN PRODUCT

If two tables in a join query have no join condition, Database returns their Cartesian product. Oracle combines each row of one table with each row of the other. A Cartesian product always generates many rows and is rarely useful. For example, the Cartesian product of two tables, each with 100 rows, has 10,000 rows. Always include a join condition unless you specifically need a Cartesian product. Cartesian product of tables EMP and DEPT will have 14x4=56 rows.  

 

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