Relational Operator - Join

join is a relational operator that pair tuples (row) of two relations (table, …) according to a condition (predicate).

<MATH> \LARGE R_1 \bowtie_c R_2 </MATH> where:

• c is a condition (ie predicate)

See SQL - Joins (Home)

<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

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

• Relational Operator - Hash Joins
• Relation Operator - Nested Loop Join