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•  Relation state, Relation instance: Other names for the  entity set.
                  •  Cardinality: Numbers of tuples in the entity set.
                  •  Order within a table:

                    (i)  Rows in a table are considered unordered.
                    (ii)  Columns in a table are considered unordered.
                  NULL: When value of an attribute is unknown, it is denoted by NULL.
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                  Candidate Key: The minimal set of attributes that identifies a tuple in a table is called its candidate key.
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                  Primary Key: It is the column(s) that contain values that uniquely identifes each row in a table/relation. Any
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                  one of the candidate keys can be chosen as the primary key.
                  Entity Integrity Constraint: No primary key attribute can have a NULL value.
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                  Alternate Key: A candidate keys that is not chosen as primary key.
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                  Foreign Key: A non-key attribute of table R1 that is part of primary key of table R2 is a foreign key of table
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                  R1.
                  Foreign Key Constraint: If a table R1 has a foreign key (say, A)which is part of the primary key of another
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                  table R2, then
                  •  the value of foreign key A maybe be NULL.
                  •  the value of foreign key A in table R1 maybe be non- NULL (say, val). In this case. There must be a tuple

                    in table R2 having val as the value of attribute B.
                  NOT NULL Constraint: It applies to an attribute. No row in the table can have a NULL value of the specified
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                  attribute. For example, if we specify NOT NULL constraint for the Salary attribute of EMPLOYEE table,
                  then Salary of an employee cannot have the value NULL.
                  Uniqueness Constraint: It applies to an attribute. No two rows can have same value of the specified attribute.
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                  For example, in a STUDENT table, AadharNo may be defined to be UNIQUE, even if it is not the primary key.
                  Domain Constraint: Domain Constraint allows you to restrict the set of values that a particular attribute  may
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                  take. For example, we may constraint  attribute Salary to  take values between 8000 and 100000.
                  SQL: Most popular language for creating and manipulating relational databases.
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                  DDL: Used for defining database attributes, their types, and constraints on attribute values.
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                  DML: Used to retrieve and modify data.
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                  Data Types:
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                  •  INT or INTEGER:  32-bit integer.
                  •  SMALLINT: typically 16-bit integer.
                  •  DECIMAL(L, D): Fixed point decimal, comprising L digits,  D digits after the decimal point.
                  •  FLOAT(p): Floating point number.

                  •  CHAR(L) or CHARACTER(L): Fixed-length character strings of length L.
                  •  VARCHAR(L): Variable-length character strings of length up to L.
                  •  DATE: The standard date format is YYYY-MM-DD.
                  CREATE DATABASE database-name: creates a database and assigns it a name.
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                  SHOW DATABASES: Displays names of all the existing databases.
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                  USE database_name: makes the database active for use.
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