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 Data Type:
| 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; |
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