What are joins and types, number of cols after all types of joins...

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Joins are used to combine rows from two or more tables based on a related column between them.

Types of joins include inner join, outer join (left, right, full), and cross join.
Number of columns after join depends on the type of join and the columns being selected.
For example, an inner join will only include rows that have matching values in both tables, resulting in a combined table with columns from both tables.
A left outer join will include all rows from the left table and matching rows from the right table, resulting in a combined table with columns from both tables.
A cross join will combine every row from the first table with every row from the second table, resulting in a combined table with columns from both tables.

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