Relational Operator - Join
Table of Contents
About
join is a relational operator that pair tuples (row) of two relations (table, …) according to a condition (predicate).
Articles Related
Notation
Relational
<MATH> \LARGE R_1 \bowtie_c R_2 </MATH> where:
- c is a condition (ie predicate)
SQL
Symbol
Rule
<MATH> \LARGE R_1 \bowtie_{ predicate } R_2 = \sigma_{ predicate } ( R_1 \times R_2 ) </MATH> which means:
- the join of R1 and R2 on a predicate
is the same than
- the selection of the cross product of R1 and R2 on the same predicate
Type
Logical
Short hand:
- Relational Operator - Equi-joins - joins where the condition is only an equality
- Relational Algebra - Theta-join - joins where the condition is not an equality