8/10/2023 0 Comments Postgresql cross join![]() ![]() VALUES(2, 'My Second SQL book','John Mayer', NOW()) INSERT INTO reviews (id, book_id, user_id, review_content) ![]() INSERT INTO users_books (user_id, book_id, checkout_date) VALUES(1, 'My First SQL book', 'Mary Parker', NOW()) ![]() INSERT INTO books (id, title, author, published_date) VALUES (1, '1 Market Street', 'San Francisco', 'CA') INSERT INTO addresses (user_id, street, city, state) INSERT INTO users (id, username) VALUES (1, 'John Smith') ![]() John Smith also has 2 reviews for each of the books. John Smith has an address at 1 Market Street, San Francisco, CA, and has checked out 2 books: My First SQL Book and My Second SQL Book. The first set of statements set up the data for the first user, John Smith. Book: My Second SQL Book by John Mayer with a review My Second Review.Book: My First SQL Book by Mary Parker with a review My First Review.Address: 1 Market Street, San Francisco, CA.If you haven't run the statements, you can follow along and run them now as we explain the data and statements used to create them. Let's take a look at how these statements work. The table data can also be set up by running the following command in addition to the command at the top of the page. This mix of data lets us explore the various types of JOIN, and observe the results of queries. To set up the data, we will create 3 users, 3 books and 2 reviews in our library database by running the SQL commands shown below. To describe these different types, we need to add some data to our database so we can show the characteristics of each type. A table doesn't have to contain all the fields related to its rows, and table data can be used in multiple places. With JOINs, relational databases can reduce redundancy. JOINs are clauses in SQL statements that link two tables based on one or more fields. SQL handles relationships through the use of JOINs. Psql -d library < joins.sql What is a SQL Join? You can create those tables manually or restore the database from a backup file with the following command: The library database is needed to work through this chapter's content.įor this chapter, we will only need to start with the schema of the various tables we've created With the lateral join the work data is reduced to one row in the subselect query.If you don't already have a database named "library", then make sure to run createdb library from the terminal before starting this chapter. For large tables this requires a lot of temporarily memory. Otherwise we would have need to join the orders and orderlines table and do the aggregation at the end. Depending on the use case this can increase performance. Notice that with the LATERAL join keyword we can reference the orders table in a subselect query which is not possible with standard joins. ORDER BY orderlinestatus.weight DESC LIMIT 1ĪS subquery_statushigh ON (subquery_statushigh.orderid=orders.orderid) SELECT orderlinestatus.name AS statushigh, orderlines.orderid GROUP BY orderlines.orderid, orderlinestatus.name, orderlinestatus.weightĪS subquery_statuslow ON (subquery_statuslow.orderid=orders.orderid), SELECT orderlinestatus.name AS statuslow, orderlines.orderidįROM orderlines JOIN orderlinestatus USING(orderlinestatusid) WHERE orderlines.orderid = orders.orderid SELECT orders.orderid, statuslow, statushigh We have a table with orders and want to join a related table orderlines to find out the lowest and highest status of the order lines. This is repeated for each row or set of rows from the column source table(s). The resulting row(s) are joined as usual with the rows they were computed from. When a FROM item contains LATERAL cross-references, evaluation proceeds as follows: for each row of the FROM item providing the cross-referenced column(s), or set of rows of multiple FROM items providing the columns, the LATERAL item is evaluated using that row or row set's values of the columns. In the latter case it can also refer to any items that are on the left-hand side of a JOIN that it is on the right-hand side of. Table functions appearing in FROM can also be preceded by the key word LATERAL, but for functions the key word is optional the function's arguments can contain references to columns provided by preceding FROM items in any case.Ī LATERAL item can appear at top level in the FROM list, or within a JOIN tree. (Without LATERAL, each subquery is evaluated independently and so cannot cross-reference any other FROM item.) This allows them to reference columns provided by preceding FROM items. In the PostgreSQL documentation: Subqueries appearing in FROM can be preceded by the key word LATERAL. PostgreSQL supports the SQL join type: LATERAL. ![]()
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