MIST 4610 Chapter Notes - Chapter 5: Associative Entity, Universal Quantification
9 Feb 2018
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Watson- Chapter 5 | The Many-to-
Many Relationship
• When we have an m:m relationship, we create a 3rd
entity to link the entities through two 1:m
relationships
o Associative entity: the third entity
o Weak entity: relies on another entity for its
existence and identification
• Identifying relationship: shown by a solid line; used
when the entity at the many end of the relationship is
a weak entity and needs the identifier of the one end
of the relationship to uniquely identify an instance of
the relationship
o An identifying relationship corresponds to the +
sign associated w/ a crow's foot
o Dashed line shows a non-identifying relationship
• The join operation can extend from two tables to
three or more by specifying the tables to be joined +
the matching conditions
• EXISTS is used in the WHERE clause to test whether a
table contains at least one row satisfying a specified
condition
o Returns the value 'true' if and only if some row
satisfied the condition; otherwise it returns 'false'
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