A Guide to SQL, Seventh Edition

Objectives Use joins to retrieve data from more than one table Use the IN and EXISTS operators to query multiple tables Use a subquery within a subquery Use an alias A Guide to SQL, Seventh Edition

Objectives Join a table to itself Perform set operations (union, intersection, and difference) Use the ALL and ANY operators in a query Perform special operations (inner join, outer join, and product) A Guide to SQL, Seventh Edition

Querying Multiple Tables When querying more than one table, the tables must be joined Join tables by finding columns with matching data Join tables by using a condition in the WHERE clause A Guide to SQL, Seventh Edition

Joining Two Tables In the SELECT clause, list all columns you want to display In the FROM clause, list all tables involved in the query In the WHERE clause, restrict to the rows that have common values in matching columns A Guide to SQL, Seventh Edition

Comparing JOIN, IN, and EXISTS Tables can be joined using IN or EXISTS clause Use IN operator with a subquery Use the EXISTS operator to retrieve data from more than one table A Guide to SQL, Seventh Edition

Correlated Subquery Subquery involves a table listed in the outer query In Figure 4.7 the ORDERS table, listed in the FROM clause of the outer query, is used in the subquery You need to qualify ORDER_NUM column in subquery as ORDERS.ORDER_NUM A Guide to SQL, Seventh Edition

Correlated Subquery For each row in the ORDERS table Subquery executed using the value of ORDERS.ORDER_NUM that appears in the row The inner query makes a list of rows in the ORDER_LINE table Where ORDER_LINE.ORDER_NUM matches this value and In which PART_NUM is equal to DR93 A Guide to SQL, Seventh Edition

Using a Subquery within a Subquery A nested subquery is a subquery within a subquery SQL evaluates the queries from the innermost query to the outermost It is possible that there is more than one approach to formulation of the queries Many DMBS have optimizers that analyze queries for efficiency A Guide to SQL, Seventh Edition

Using an Alias An alias is an alternate name for a table Used when tables are listed in the FROM clause Created by typing the name of the table, hitting a space, then typing the name of the alias Allows for simplicity A Guide to SQL, Seventh Edition

Joining a Table to Itself A self-join is when you are joining a table to itself A second use for using an alias Used when comparing records within one table Alias allows you to treat one table as two separate tables A Guide to SQL, Seventh Edition

Using a Self-Join on a Primary Key It is possible to create a self-join that involves the primary key of the table Just as in previous examples, you would list the table twice in the FROM clause with aliases A Guide to SQL, Seventh Edition

Joining Several Tables Condition shows how the columns are related for each pair of tables A Guide to SQL, Seventh Edition

Joining Several Tables Step-By-Step In the SELECT clause list all the columns to display Qualify the column name if needed In the FROM clause list all tables Include tables used in the WHERE clause, even if they are not in the SELECT clause A Guide to SQL, Seventh Edition

Joining Several Tables Step-By-Step Take one pair of related tables at a time Indicate in the WHERE clause the condition that relates the tables Join conditions with the AND operator Include any additional conditions in the WHERE clause Connect them with the AND operator A Guide to SQL, Seventh Edition

Set Operations Set operations are used for taking the union, intersection, and differences of two tables The union of two tables is a table containing every row that is in either the first table, the second table, or both tables A Guide to SQL, Seventh Edition

Set Operations The intersection (intersect) of two tables is a table containing all rows that are in both tables The difference (minus) of two tables is the set of all rows that are in the first tables but are not in the second table A Guide to SQL, Seventh Edition

Restrictions to Set Operations Requirements for tables to be union compatible Have the same number of columns AND Their corresponding columns have identical data types and lengths A Guide to SQL, Seventh Edition

Special Operations Inner Join A join that compares the tables in the FROM clause and lists on those rows that satisfy the condition in the WHERE clause Outer Join A join that lists all the rows from one of the tables in a join, regardless of matching A Guide to SQL, Seventh Edition

Outer Joins Left outer join: all rows from the table on the left (listed first in the query) will be included; matching rows only from the table on the right will be included Right outer join: all rows from the table on the right will be included; matching rows only from the table on the left will be included Full outer join: all rows from both tables will be included regardless of matches A Guide to SQL, Seventh Edition

Product The product (Cartesian Product) of two tables is the combination of all rows in the first table and all rows in the second table Omit the WHERE clause to form a product A Guide to SQL, Seventh Edition

Summary Join tables with various methods A subquery can contain another subquery An alias can be used to simplify the SQL command as well to create self join UNION, INTERSECT, MINUS commands are introduced To form a product of two tables, include both tables in the FROM clause and omit the WHERE clause A Guide to SQL, Seventh Edition