Learn Database Concepts

SELECT   E.EMPNO,   E.ENAME,   E.JOB,   D.DEPTNO,   D.LOC,   E.SAL FROM   scott.emp E LEFT JOIN SCOTT.DEPT D ON   E.DEPTNO=D.DEPTNO; SELECT   E.EMPNO,   E.ENAME,   E.JOB,   D.DEPTNO,   D.LOC,   E.SAL,   (     SELECT      grade     FROM       SCOTT.SALGRADE S     WHERE       E.SAL BETWEEN S.LOSAL AND S.HISAL   ) AS SALGRADE FROM   scott.emp E LEFT JOIN SCOTT.DEPT D ON   E.DEPTNO=D.DEPTNO;

lets assume that there was a table ABC that was already existing in the database and you want to add an additional column with unique primary key values. sql to create table ABC :   CREATE TABLE "ABC"    (    "USERNAME" VARCHAR2(30 BYTE) NOT NULL ENABLE,     "USER_ID" NUMBER NOT NULL ENABLE,     "CREATED" DATE NOT NULL ENABLE )    TABLESPACE "USERS" ; Now we  can add an additional column ID which will be populated with all unique values for PrimaryKey. alter table abc add(ID NUMBER); We will now create a sequence and get the values from the seq and insert them into table ABC new ID column: CREATE SEQUENCE SEQ_ID START WITH 1 INCREMENT BY 1 MAXVALUE 999999 MINVALUE 1 NOCYCLE; Now insert the unique values into the database with below sql: UPDATE abc SET ID = SEQ_ID.NEXTVAL; now you can add unique constraint (or) add primary key constraint to table,so that it wont take any more duplicate value into the table. alter ...