Oracle SQL Part 3

Oracle Database, SQL
Day 3
Trainers:
Gurpreet Singh

Oracle Database, SQL2
Day 3 Agenda

Joins
An SQL JOIN clause combines columns from one or more tables in a relational database. It creates a set that can
be saved as a table or used as it is. A JOIN is a means for combining columns from one (self-table) or more
tables by using values common to each.
Let’s say we have two sets of data in our relational database: table A and table B, with some sort of relation
specified by primary and foreign keys. The result of joining these tables together can be visually represented by
the following diagram:

Joins
The extent of the overlap, if any, is determined by how many records in Table A match the records in Table B.
Depending on what subset of data we would like to select from the two tables, the four join types can be
visualized by highlighting the corresponding sections of the Venn diagram:

JoinsImagine you’re running a store and would like to record information about your customers and their orders. By
using a relational database, you can save this information as two tables that represent two distinct entities:
customers and orders.
CUSTOMERS
ORDERS

Joins
Note that the orders table contains two keys: one for the order and one for the customer who placed that
order. In scenarios when there are multiple keys in a table, the key that refers to the entity being described
in that table is called the Primary Key (PK) and other key is called a Foreign Key (FK).
In our example, order_id is a primary key in the orders table, while customer_id is both a primary key in the
customers table, and a foreign key in the orders table. Primary and foreign keys are essential to describing
relations between the tables, and in performing SQL joins.

Sub Queries
A sub query is a SQL query nested inside a larger query.
A sub query may occur in :
- A SELECT clause
- A FROM clause
- A WHERE clause
The sub query can be nested inside a SELECT, INSERT, UPDATE, or DELETE statement or inside another sub
query.
A sub query is usually added within the WHERE Clause of another SQL SELECT statement.
You can use the comparison operators, such as >, <, or =. The comparison operator can also be a multiple-
row operator, such as IN, ANY, or ALL.
A sub query is also called an inner query or inner select, while the statement containing a sub query is also
called an outer query or outer select.
The inner query executes first before its parent query so that the results of an inner query can be passed to
the outer query.
You can use a sub query in a SELECT, INSERT, DELETE, or UPDATE statement to perform the following tasks:
 Compare an expression to the result of the query.
 Determine if an expression is included in the results of the query.
 Check whether the query selects any rows.

Sub Queries
• The sub query (inner query) executes once before the main query (outer query)
executes.
• The main query (outer query) use the sub query result.

Sub Queries
There are some guidelines to consider when using sub queries :
 A sub query must be enclosed in parentheses.
 A sub query must be placed on the right side of the comparison operator.
 Sub queries cannot manipulate their results internally, therefore ORDER BY
clause cannot be added into a sub query. You can use an ORDER BY clause in
the main SELECT statement (outer query) which will be the last clause.
 Use single-row operators with single-row sub queries.
 If a sub query (inner query) returns a null value to the outer query, the outer
query will not return any rows when using certain comparison operators in a
WHERE clause.

Sub Queries
Types of Sub Queries:
 Single row sub query : Returns zero or one row.
 Multiple row sub query : Returns one or more rows.
 Multiple column sub queries : Returns one or more columns.
 Correlated sub queries : Reference one or more columns in the outer SQL
statement. The sub query is known as a correlated sub query because the sub
query is related to the outer SQL statement.
 Nested sub queries : Sub queries are placed within another sub query.

Sub Queries
A single row sub query returns zero or one row to the outer SQL statement. You can place
a sub query in a WHERE clause, a HAVING clause, or a FROM clause of a SELECT
statement.

Sub Queries
Multiple row sub query returns one or more rows to the outer SQL statement. You may use
the IN, ANY, or ALL operator in outer query to handle a sub query that returns multiple
rows.
You can use the ANY operator to compare a value with any value in a list. You must place
an =, <>, >, <, <= or >= operator before ANY in your query. The following example uses
ANY to check if any of the agent who belongs to the country 'UK'.

Sub Queries
Multiple column sub query returns more than one columns to the outer SQL statement.

Sub Queries
SQL Correlated Sub queries are used to select data from a table referenced in the outer
query. The sub query is known as a correlated because the sub query is related to the outer
query. In this type of queries, a table alias (also called a correlation name) must be used to
specify which table reference is to be used.
The alias is the pet name of a table which is brought about by putting directly after the
table name in the FROM clause. This is suitable when anybody wants to obtain information
from two separate tables.

Sub Queries
A sub query can be nested inside other sub queries. SQL has an ability to nest queries
within one another. A sub query is a SELECT statement that is nested within another
SELECT statement and which return intermediate results. SQL executes innermost sub
query first, then next level.

CONTACTS
21
Gurpreet Singh
Senior Software Engineer
CUG. 2142
M. +91-9803723925
Gurpreet.singh2@soprasteria.com
Oracle Database, SQL

Oracle SQL Part 3

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Oracle SQL Part 3