AFAIK it is not possible to define a check constraint that makes counts of the table records. I think you will need to put this validations on the data entering application, and I would suggest this value (2) being stored on the database, so the app doesn't need to be modified if this policy changes. Other approach could be to add a new column (subjects_count) which will be updated each time a new row is added (or an update is made). This new column could have a check constraint to only allow values not greater than 2 (<=2), so, the update to this column will fail if it is the third row for a given professor. Hope this helps. *********************************** Using this technique you could also add a unique constraint to the prof_name and the subjects_count columns so that data can not be entered in more than once. *********************************** The CREATE TABLE script in ORACLE would look something like this: <pre>create table professor_subject ( prof_name varchar2(20), subject_code varchar2(5), dept varchar2(5), subjects_count number(5) constraint chk_profesorsubject_subjcount check (subjects_count <= 2), constraint unq_professorsubject_pn_sjc unique (prof_name,subjects_count));</pre>

AFAIK it is not possible to define a check constraint that makes counts of the table records. I think you will need to put this validations on the data entering application, and I would suggest this value (2) being stored on the database, so the app doesn't need to be modified if this policy changes. Other approach could be to add a new column (subjects_count) which will be updated each time a new row is added (or an update is made). This new column could have a check constraint to only allow values not greater than 2 (<=2), so, the update to this column will fail if it is the third row for a given professor. Hope this helps. *********************************** Using this technique you could also add a unique constraint to the prof_name and the subjects_count columns so that data can not be entered in more than once. *********************************** The CREATE TABLE script in ORACLE would look something like this: <pre>create table professor_subject ( prof_name varchar2(20), subject_code varchar2(5), dept varchar2(5), subjects_count number(5) constraint chk_profesorsubject_subjcount check (subjects_count <= 2), constraint unq_professorsubject_pn_sjc unique (prof_name,subjects_count));</pre>