## 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