What is a nested query in Oracle?
A subquery in the WHERE clause of a SELECT statement is also called a nested subquery. A subquery can contain another subquery. Oracle Database imposes no limit on the number of subquery levels in the FROM clause of the top-level query. You can nest up to 255 levels of subqueries in the WHERE clause.
Can we use alias name in WHERE clause in Oracle?
You can use the alias in GROUP BY, ORDER BY, or HAVING clauses to refer to the column. Standard SQL disallows references to column aliases in a WHERE clause. This restriction is imposed because when the WHERE clause is evaluated, the column value may not yet have been determined.
How do I create a sub query in Oracle?
In Oracle, a subquery is a query within a query. You can create subqueries within your SQL statements. These subqueries can reside in the WHERE clause, the FROM clause, or the SELECT clause.
Do you need As for alias in SQL?
In Standard SQL they can have an optional AS keyword but it should not be used because it may give the impression that something is being renamed when it is not. In fact, it should be omitted to enforce the point that it is a range variable.
WHERE is nested query?
A Subquery or Inner query or a Nested query is a query within another SQL query and embedded within the WHERE clause. A subquery is used to return data that will be used in the main query as a condition to further restrict the data to be retrieved.
Can we write multiple query inside the subquery?
Multiple row subquery 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 subquery that returns multiple rows.
What is alias in Oracle SQL?
Oracle ALIASES can be used to create a temporary name for columns or tables. TABLE ALIASES are used to shorten your SQL to make it easier to read or when you are performing a self join (ie: listing the same table more than once in the FROM clause).
Can we use alias in where clause?
In PROC SQL, a column alias can be used in a WHERE clause, ON clause, GROUP BY clause, HAVING clause, or ORDER BY clause. In the ANSI SQL standard and ISO SQL standard, the value that is associated with a column alias does not need to be available until the ORDER BY clause is executed.
What is an alias command in SQL?
SQL aliases are used to give a table, or a column in a table, a temporary name. Aliases are often used to make column names more readable. An alias only exists for the duration of that query. An alias is created with the AS keyword.
How to change the alias in the subquery to PC2?
Changing the alias in the subquery to say PC2would at least make the query logically easier to read. SELECT PC.ID ,( SELECT PC2.NAME from PRODUCTS_CATEGORIES PC2 where PC.PARENT = PC2.ID ) ,PC.NAME FROM PRODUCTS_CATEGORIES PC
What is a nested subquery in SQL?
A subquery nested in the WHERE clause of the SELECT statement is called a nested subquery. A subquery can contain another subquery. Oracle allows you to have an unlimited number of subquery levels in the FROM clause of the top-level query and up to 255 subquery levels in the WHERE clause.
How does Oracle evaluate the whole query above?
Oracle evaluates the whole query above in two steps: First, execute the subquery. Second, use the result of the subquery in the outer query. A subquery which is nested within the FROM clause of the SELECT statement is called an inline view. Note that other RDBMS such as MySQL and PostgreSQL use…
What is outer query in subquery in Oracle?
Second, the outer query returns the customers with the id that are not in the list returned by the subquery. In this tutorial, you have learned about the Oracle subquery which gives you an alternative way to construct more readable queries without using complex joins or unions.