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Class 11 Database Concepts: cheat sheet
CBSE Informatics Practices. Why databases replace file systems, the relational model (relation, tuple, attribute, domain, degree and cardinality) and keys: candidate, primary, alternate, composite and foreign.
Key ideas
- Separate files cause redundancy (the same data stored many times), inconsistency (copies that disagree), data isolation, dependence on file layouts and no control over who may read or change what.
- A DBMS is software to create, store, manage and query databases while controlling access; MySQL is one.
- In the relational model a table is a relation, a row a tuple, a column an attribute, and the set of allowed values of an attribute its domain.
- Degree is the number of attributes (columns); cardinality is the number of tuples (rows).
- A candidate key has a different, non-NULL value in every tuple; the one chosen is the primary key and the others are alternate keys.
- When no single attribute is unique, two or more together form a composite primary key, such as (AttDate, RollNo) in an attendance table.
- A foreign key refers to the primary key of another relation; referential integrity means each of its values must exist there (or be NULL).
Remember
| Degree | the number of columns (attributes) |
| Cardinality | the number of rows (tuples); after adding a rows and deleting d: n + a − d |
| Primary key | unique and never NULL in every row |
| Alternate keys | all the candidate keys except the primary key (one fewer) |
| Foreign key check | look up each value in the referenced table; a missing value breaks referential integrity |
| Schema; instance | the structure of the relations; the data stored at one moment |
Watch out
- Swapping degree and cardinality: degree counts columns.
- Choosing Name or City as a key: their values can repeat.
- Counting the primary key among the alternate keys.
- Expecting one attribute of an attendance table to be unique: only the pair of date and roll number is.
Worked example
Table: ATTENDANCE AttDate | RollNo | Status 2026-07-12 | 9 | P 2026-07-12 | 34 | P 2026-07-13 | 9 | P 2026-07-13 | 34 | P 2026-07-14 | 9 | P 2026-07-14 | 34 | P Answer Yes or No: (a) Is AttDate alone unique? (b) Is RollNo alone unique? (c) Is the pair (AttDate, RollNo) unique?
Working: each date repeats for every student and each roll number for every date; the pair appears once
Answer: (a) No; (b) No; (c) Yes
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