You can combine rows from two tables with a join. This tutorial explains the join methods using these two tables:
select * from toys; select * from bricks;
Oracle Database has two syntaxes for joining tables. The proprietary Oracle method. And the ANSI standard way.
Oracle syntax joins tables in the where clause. ANSI style has a separate join clause. This tutorial will show both methods.
We recommend you use ANSI syntax. This clearly separates the join and filter clauses. This can make your query easier to read, particularly with outer joins. But which approach you use is largely a matter of personal preference.
Whatever you do, choose one style and stick with it!
A cross join returns every row from the first table matched to every row in the second. This will always return the Cartesian product the two table's rows. I.e. the number of rows in the first table times the number in the second.
Toys and bricks both store three rows. So cross joining them returns 3 * 3 = 9 rows.
To cross join tables using Oracle syntax, simply list the tables in the from clause:
select * from toys, bricks;
Using ANSI style, type cross join between the tables you want to combine:
select * from toys cross join bricks;
It's rare you'll use cross joins in your code. If you need to use cross joins often it's a sign something is wrong with your data model! If you do need a cross join, ANSI syntax makes it clear this is your intent. And not that you forgot your join clause!
An inner join (or just join) links two tables. It compares values in one or more columns from each. It only returns rows which match the join conditions in both tables.
The simplest join checks if the values in a column from one table equal the values in a column from the other.
For example, you can join toys and bricks where the toy_id equals the brick_id. Only the ids 2 & 3 are in both tables. And there is only one row for each value in each table. So this join returns two rows:
select * from toys, bricks where toy_id = brick_id;
Using the ANSI way, the join criteria go in the on clause.
select * from toys inner join bricks on toy_id = brick_id;
An inner join can return any number of rows from zero to the Cartesian product of the two tables.
If you join on a column that has repeated values, you'll get many copies of the matching rows in the joined table. For example, there are two blue bricks. The toy Cuteasaurus is blue. So joining the tables on colour returns two copies of the row for Cuteasaurus:
select * from toys join bricks on toy_colour = brick_colour;
Note that the keyword inner is optional.
You can also join on inequalities. For example, to return all the rows that have different colours, write:
select * from toys join bricks on toy_colour <> brick_colour;
Note there are no pink bricks. So this returns a copy of the pink toy (Miss Snuggles) for each row in bricks. So Miss Snuggles appears three times.
Complete the query below to:
select * from toys join bricks /* TODO */
The query should return the following row:
TOY_ID TOY_NAME TOY_COLOUR BRICK_ID BRICK_COLOUR BRICK_SHAPE 3 Baby Turtle green 2 blue cube
An outer join returns all the rows from one table along with the matching rows from the other. Rows without a matching entry in the outer table return null for the outer table's columns.
An outer join can either be left or right. The direction defines which side of the join the database preserves the rows for.
The following left joins toys and bricks on their IDs. Toys is on the left side of the join. So the database returns all its rows. You also get the rows from bricks which have the same id as these:
select * from toys left outer join bricks on toy_id = brick_id;
To outer join with Oracle syntax use the (+) operator. This goes after the columns of the table you want to optionally include. So the following is the same as the previous query:
select * from toys, bricks where toy_id = brick_id (+);
To return all the rows from bricks, you can switch from a left join to a right. Or change the order of the tables in your query:
select * from toys right join bricks on toy_id = brick_id; select * from bricks left join toys on toy_id = brick_id;
To do the same with Oracle syntax, move the plus to the columns of the toys table:
select * from toys, bricks where toy_id (+) = brick_id;
An outer join will always return all the rows from the preserved table. So the number of rows returned varies from the size of the preserved table to the Cartesian product of the tables.
You can filter the rows returned by a join in your where clause. For example, the following joins the tables on their id. Then filters the result to only show those combinations that have a green brick:
select * from toys join bricks on toy_id = brick_id where brick_colour = 'green';
But you must take care when doing this with outer joins!
Say you want to return all the toy rows. And any brick rows with a matching id and the colour green. So you expect your results to include the three rows from toys. But only brick details for brick_id = 3:
TOY_ID TOY_NAME TOY_COLOUR BRICK_ID BRICK_COLOUR BRICK_SHAPE 3 Baby Turtle green 3 green cube 1 Miss Snuggles pink <null> <null> <null> 2 Cuteasaurus blue <null> <null> <null>
If you filter the outer joined table in your where clause, you'll convert the query to an inner join. And only get one row:
select * from toys left join bricks on toy_id = brick_id where brick_colour = 'green'; select * from toys, bricks where toy_id (+) = brick_id and brick_colour = 'green';
To fix this, you must place the filtering criteria for the outer joined table in the join clause. And in Oracle style, add (+) to all the outer joined table's columns:
select * from toys left join bricks on toy_id = brick_id and brick_colour = 'green'; select * from toys, bricks where toy_id = brick_id (+) and brick_colour (+) = 'green';
Complete the query below to return:
select * from bricks left join toys /* TODO */
The query should return the following rows:
BRICK_ID BRICK_COLOUR BRICK_SHAPE TOY_ID TOY_NAME TOY_COLOUR 2 blue cube 2 Cuteasaurus blue 3 green cube <null> <null> <null> 4 blue pyramid <null> <null> <null>
Sometimes you may want to join two tables to find the matching rows. But also include any unmatched rows from both tables. I.e. a "double outer" join. This is known as a full (outer) join.
You do this using ANSI syntax like so:
select * from toys full join bricks on toy_id = brick_id;
Writing a full outer join in Oracle syntax is a little clumsy. You need two outer join queries. One for each table. Then to combine the results of these using union all:
select * from toys, bricks where toy_id = brick_id (+) union all select * from toys, bricks where toy_id (+) = brick_id and toy_id is null;
Like cross joins, it's rare to use this. But it's useful to have in your SQL toolkit!