What is inner query and outer query in SQL?

A subquery is a query within another query. The outer query is called as main query and inner query is called as subquery. The subquery generally executes first, and its output is used to complete the query condition for the main or outer query. Subquery must be enclosed in parentheses.

What is inner query in SQL?

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.

What is the difference between inner query and outer query?

When a query is included inside another query, the Outer query is known as Main Query, and Inner query is known as Subquery. In Nested Query, Inner query runs first, and only once. Outer query is executed with result from Inner query. Hence, Inner query is used in execution of Outer query.

Can inner query use outer query?

A subquery is also called an inner query or inner select, while the statement containing a subquery 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.

What is the difference between query and subquery?

A query is an operation that retrieves data from one or more tables or views. In this reference, a top-level SELECT statement is called a query, and a query nested within another SQL statement is called a subquery.

Inline query, inline view, subquery, inner query, nested query and correlated query in SQL

Which join is faster in SQL?

If you dont include the items of the left joined table, in the select statement, the left join will be faster than the same query with inner join. If you do include the left joined table in the select statement, the inner join with the same query was equal or faster than the left join.

What is SQL Indexing?

A SQL index is used to retrieve data from a database very fast. Indexing a table or view is, without a doubt, one of the best ways to improve the performance of queries and applications. A SQL index is a quick lookup table for finding records users need to search frequently.

What is nested query?

A nested query is a query that has another query embedded within it. The embedded query is called a subquery. A subquery typically appears within the WHERE clause of a query. It can sometimes appear in the FROM clause or HAVING clause.

What is the difference between any and all operators?

The ANY and ALL operators are used with a WHERE or HAVING clause. The ANY operator returns true if any of the subquery values meet the condition. The ALL operator returns true if all of the subquery values meet the condition.

What is correlation in SQL?

In a SQL database query, a correlated subquery (also known as a synchronized subquery) is a subquery (a query nested inside another query) that uses values from the outer query. Because the subquery may be evaluated once for each row processed by the outer query, it can be slow.

How many types of JOINs?

There are four main types of JOINs in SQL: INNER JOIN, OUTER JOIN, CROSS JOIN, and SELF JOIN.

What is difference union and union all in SQL?

The only difference between Union and Union All is that Union extracts the rows that are being specified in the query while Union All extracts all the rows including the duplicates (repeated values) from both the queries.

Which is called as a virtual table in SQL?

In SQL, a view is a virtual table based on the result-set of an SQL statement. A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database.

How do I write an outer query in SQL?

Subqueries can be used with SELECT, UPDATE, INSERT, DELETE statements along with expression operator. It could be equality operator or comparison operator such as =, >, =, <= and Like operator. A subquery is a query within another query. The outer query is called as main query and inner query is called as subquery.

What is cursor in SQL?

A cursor holds the rows (one or more) returned by a SQL statement. The set of rows the cursor holds is referred to as the active set. You can name a cursor so that it could be referred to in a program to fetch and process the rows returned by the SQL statement, one at a time.

What are triggers in SQL?

A trigger is a special type of stored procedure that automatically runs when an event occurs in the database server. DML triggers run when a user tries to modify data through a data manipulation language (DML) event. DML events are INSERT, UPDATE, or DELETE statements on a table or view.

Is null statement in SQL?

Description. The IS NULL condition is used in SQL to test for a NULL value. It returns TRUE if a NULL value is found, otherwise it returns FALSE. It can be used in a SELECT, INSERT, UPDATE, or DELETE statement.

What is natural join in SQL?

A NATURAL JOIN is a JOIN operation that creates an implicit join clause for you based on the common columns in the two tables being joined. Common columns are columns that have the same name in both tables. A NATURAL JOIN can be an INNER join, a LEFT OUTER join, or a RIGHT OUTER join. The default is INNER join.

IS NULL operator in SQL?

SQL IS NULL operator is used to checking if the value of a column is null or not. The operator filters the result set based on the null column value.

What are constraints in SQL?

SQL constraints are used to specify rules for the data in a table. Constraints are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the table. If there is any violation between the constraint and the data action, the action is aborted.

What is aggregate function in SQL?

An aggregate function performs a calculation on a set of values, and returns a single value. Except for COUNT(*) , aggregate functions ignore null values. Aggregate functions are often used with the GROUP BY clause of the SELECT statement.

What is WHERE clause in SQL?

In a SQL statement, the WHERE clause specifies criteria that field values must meet for the records that contain the values to be included in the query results. For an overview of Access SQL, see the article Access SQL: basic concepts, vocabulary, and syntax.

What is schema in SQL?

In a SQL database, a schema is a list of logical structures of data. A database user owns the schema, which has the same name as the database manager. As of SQL Server 2005, a schema is an individual entity (container of objects) distinct from the user who constructs the object.

What is primary key SQL?

The PRIMARY KEY constraint uniquely identifies each record in a table. Primary keys must contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).

What are keys in SQL?

An SQL key is either a single column (or attribute) or a group of columns that can uniquely identify rows (or tuples) in a table. SQL keys ensure that there are no rows with duplicate information. Not only that, but they also help in establishing a relationship between multiple tables in the database.