In this section, we will try to explain about important database ALTER commands that are used by a data modeler by relating it with our example data.
ALTER TABLE – Add column:
ALTER TABLE EMPLOYEE_DTL ADD JOIN_DATE DATE;
ALTER TABLE – Rename column:
ALTER TABLE EMPLOYEE_DTL RENAME column JOIN_DATE TO EMP_JOIN_DT;
ALTER TABLE – Modify column’s datatype:
ALTER TABLE EMPLOYEE_DTL MODIFY EMP_JOIN_DT VARCHAR2(10);
ALTER TABLE – Drop column:
ALTER TABLE EMPLOYEE_DTL DROP COLUMN EMP_JOIN_DT;
ALTER TABLE – Add check constraint:
ALTER TABLE EMPLOYEE_DTL ADD CONSTRAINT CH_SAL CHECK(SLRY_AMT BETWEEN 4000 AND 7000);
ALTER TABLE – Add unique constraint:
ALTER TABLE EMPLOYEE_DTL ADD CONSTRAINT UN_SSN UNIQUE(SSN);
ALTER TABLE – Disable/Enable/Drop constraint:
ALTER TABLE EMPLOYEE_DTL DISABLE CONSTRAINT UN_SSN;
ALTER TABLE EMPLOYEE_DTL ENABLE CONSTRAINT UN_SSN;
ALTER TABLE EMPLOYEE_DTL DROP CONSTRAINT UN_SSN;
ALTER TABLE – Modify constraint:
ALTER TABLE EMPLOYEE_DTL MODIFY SLRY_AMT NUMBER(7,2) NULL;
ALTER TABLE EMPLOYEE_DTL MODIFY SLRY_AMT NUMBER(7,2) NOT NULL;