Database Administration Questions

A Subquery or Inner query or a Nested query is a query within another SQL query and embedded within the WHERE clause.

Subqueries are most frequently used with the SELECT statement. The basic syntax is as follows::

SELECT column_name [, column_name ]
FROM table1 [, table2 ]
WHERE column_name OPERATOR
(SELECT column_name [, column_name ]
FROM table1 [, table2 ]
[WHERE])

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.

Subqueries can be used with the SELECT, INSERT, UPDATE, and DELETE statements along with the operators like =, <, >, >=, <=, IN, BETWEEN, etc.

There are a few rules that subqueries must follow −

1. Subqueries must be enclosed within parentheses.

2. A subquery can have only one column in the SELECT clause, unless multiple columns are in the main query for the subquery to compare its selected columns.

3. An ORDER BY command cannot be used in a subquery, although the main query can use an ORDER BY. The GROUP BY command can be used to perform the same function as the ORDER BY in a subquery.

4. Subqueries that return more than one row can only be used with multiple value operators such as the IN operator.

5. The SELECT list cannot include any references to values that evaluate to a BLOB, ARRAY, CLOB, or NCLOB.

6. A subquery cannot be immediately enclosed in a set function.

7. The BETWEEN operator cannot be used with a subquery. However, the BETWEEN operator can be used within the subquery.

The collections of entities of a particular Entity Type are grouped together into an entity set.

CTRL + ] is a computer shortcut and is used for Increase in font size.

Shortcuts :

Re-do ---- Ctrl + Y

Right align text ---- Ctrl + R

Expand or collapse the ribbon ---- Ctrl + F1

There are two IP address assigned to each device on a Wi-Fi network.

An Internal (Private) and an External (Public) IP address.

Internal IP address is used for communication between Your computer and a smartphone or whatever devices that are connected to that Wifi network. This is unique for each. If they are same the system cannot differentiate between each device.

While, the External IP address is that visible to the the internet - the websites, the internet connected Apps so on. By default, Routers with NAT (all routers today have it) assign same Public IP address to all devices under its network.

The sound is what can you hear but not touch or see.