If so, it adds in those rows placing NULLs for all the fields of the supplier. Then it starts to scan if there are any rows from orders table that are not in the result set. In RIGHT JOIN when joining table supplier with table orders PostgreSQL first does a "normal" inner join. so, for all the fields of orders it replaces NULL, here in the above output, the blank fields are NULL value. The above example shows that the row highlight in blue color, the id 0 in supplier table does not match with the table orders. Postgres=# select * from supplier left outer join orders on supplier.id=orders.id postgres=# select * from supplier left join orders on supplier.id=orders.id Thus, make sure that each row of the supplier will appear at least once in the result set. If so, it adds in those rows placing NULLs for all the fields of orders. ![]() Then it starts to scan if there are any rows from supplier table that are not in the result set. In LEFT JOIN when joining table supplier with table orders PostgreSQL first does a "normal" inner join. Here, PostgreSQL implicitly does an INNER JOIN on that column. In the above example, the 'id' column appear only once, because this column is common in both the tables. postgres=# select * from supplier natural join orders A NATURAL join is a special type of INNER join where we ask PostgreSQL to match up on all columns that have the same name. Sometimes we use the same field name for the same entity types across different tables. The criteria are boolean criteria and it must return true or false for each row combination. The above output shows that only those rows have in output who matches the criteria in both the associated table, that is why we obtain only 2 rows (1,'Pat') and (2,'Kane'). Postgres=# select * from supplier inner join orders on supplier.id=orders.id postgres=# select * from supplier join orders on supplier.id=orders.id Suppose we want to match up the supplier and order Ids. If we want to restrict the set of combinations based on some join criteria to determine which rows from both participating tables are considered to return on a match between the columns. postgres=# select * from supplier cross join orders Here in the below statement shows that the same set of results can be produced by using the CROSS JOIN terminology. Here from the above result, it seems that it takes the supplier table and combines all its rows with the first row of orders, then it takes the supplier table again and combines with the second row of orders table and finally does the same job for the third row of orders to produce 3 * 3 rows. The above result shows that the first two columns are all the fields of the first tables supplier, followed by all the fields of the second table, orders. postgres=# select * from supplier, orders ![]() The Cross Join creates a cartesian product between two sets of data. The simplest type of join is a cross join. ![]() Pictorial Presentation of PostgreSQL Joins Join keeps the structure unchanged of the base tables. A join creates a set of rows in a temporary table and works on two or more tables, and each table should at least one common field and must maintain a relation between the common fields. The Cartesian product is the set of all possible combinations between two data sets. All joins are standing on the foundation of Cartesian product. The main concept which is focusing on a join is that, two or more data sets, when joined, combined their columns into a new set of rows, including each of the columns requested from each of the data sets. How does Join works on tables in PostgreSQL?
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