difference between join and inner join code example

Example 1: only join in sql

They are functionally equivalent, but INNER JOIN can be 
a bit clearer to read, especially if the query has 
other join types (i.e. LEFT or RIGHT or CROSS) 
included in it.

Example 2: difference between inner join vs outer join

INNER JOIN:
is used when retrieving data from multiple
tables and will return only matching data.

LEFT OUTER JOIN:
is used when retrieving data from
multiple tables and will return
left table and any matching right table records.

RIGHT OUTER JOIN:
is used when retrieving data from
multiple tables and will return right
table and any matching left table records

FULL OUTER JOIN:
is used when retrieving data from
multiple tables and will return both
table records, matching and non-matching.

Example 3: two inner joins

SELECT 
  c.*, i.*, a.* 
FROM 
  invoices i 
INNER JOIN 
  client c 
ON 
  i.clientid = c.clientid 
INNER JOIN 
  address a 
ON 
  a.clientid = c.clientid 
WHERE 
  i.id = 21

Example 4: inner join vs outer join

INNER JOIN:
is used when retrieving data from multiple
tables and will return only matching data.


FULL OUTER JOIN:
is used when retrieving data from
multiple tables and will return both
table records, matching and non-matching.