Date in composite primary key column is not working for oracle

Question

Date data type in primary key is not working for oracle. Please help me to understand.

 CREATE TABLE USER_TABLE1 (
          SSN                       VARCHAR2(10 CHAR)           NOT NULL,
          DOB                       DATE                        NOT NULL,
          USER_NAME                 VARCHAR2(50)                NOT NULL,
          constraint USER_TABLE1_PK primary key(SSN,DOB)
        )
        
        insert into USER_TABLE1 values('SSN1',sysdate,'User1');
        
        select * from USER_TABLE1;

Results:

SSN DOB USER_NAME
SSN1    15-JUL-22   User1
SSN1    15-JUL-22   User1

Update:

Answer 1

In Oracle, a DATE data type is a binary data type consisting of 7 bytes representing: century, year-of-century, month, day, hour, minute and second. It ALWAYS has those components and is not stored in any particular (human-readable) format.

What you are seeing is the client application choosing to display the DATE in a format that is comprehensible to you, the user, and the implicit conversion it has used to convert the unformatted binary to a formatted string is only displaying the day-month-year components and the time components are not displayed.

If you do:

select ssn,
       TO_DATE(dob, 'YYYY-MM-DD HH24:MI:SS') AS dob,
       user_name
from   USER_TABLE1;

Then you will see the time component and see that the values in the primary key columns are not identical.

---

What you probably want is to require the time component of the DATE to always be midnight with a CHECK constraint:

CREATE TABLE USER_TABLE1 (
  SSN       VARCHAR2(10 CHAR) NOT NULL,
  DOB       DATE              NOT NULL,
  USER_NAME VARCHAR2(50)      NOT NULL,
  constraint user_table1__dob__chk CHECK (dob = TRUNC(dob)),
  constraint USER_TABLE1_PK primary key(SSN,DOB)
);

Then you cannot have DOB with the same day and different time components.

db<>fiddle here