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Database Articles
Page 261 of 547
Why in MySQL, we cannot use arithmetic operators like '=', '<' or '<>' with NULL?
The reason behind it is that we will not receive any meaningful results from the comparisons when we use NULL with the comparison operators like ‘=’, ‘ Select 10 = NULL, 10< NULL, 10NULL; +-----------+----------+----------+ | 10 = NULL | 10< NULL | 10NULL | +-----------+----------+----------+ | NULL | NULL | NULL | +-----------+----------+----------+ 1 row in set (0.07 sec)The above result set is not meaningful in any sense.
Read MoreWhat will happen to MySQL current transaction, if in the middle of that transaction, the DDL statement is executed?
The current MySQL transaction will be committed and ended when any of the DDL statement such as CREATE or DROP databases, Create, ALTER or DROP tables or stored routines is executed in the middle of the current transaction. All the database changes made in the current transaction will be made permanent and cannot be rolled back.Examplemysql> START TRANSACTION; Query OK, 0 rows affected (0.00 sec) mysql> INSERT INTO MARKS Values(6, 'Manak', 'History', 70); Query OK, 1 row affected (0.26 sec) mysql> Create table student(id int, Name Varchar(10), ); Query OK, 0 rows affected (0.84 sec)As we can see ...
Read MoreInstead of using a semicolon (;) terminator symbol, is there any other built-in-commands which execute the MySQL query?
With the help of the following built-in commands, MySQL can execute a query even if semicolon (;) terminator symbol is not used.egoWe can use this command by using \G option. It means to send the current statement to the server to be executed and display the result in vertical format. When we use \G and omitting semicolon(;) in a statement (single or multiple lines), MySQL determines the end of the statement as and when it encounters \G. Consider the example below −mysql> Select * from ratelist\G *************************** 1. row *************************** Sr: 1 Item: A Price: 502 ...
Read MoreHow can a user implicitly end current MySQL transaction?
Following are the ways with the help of which current MySQL transaction can be ended implicitly −By Running DDL statementThe current MySQL transaction will end implicitly and changes will be committed by running any of the DDL statement such as CREATE or DROP databases, Create, ALTER or DROP tables or stored routines. It is because, in MySQL, these statements cannot be rolled back.Examplemysql> START TRANSACTION; Query OK, 0 rows affected (0.00 sec) mysql> INSERT INTO MARKS Values(3, 'gaurav', 'Comp', 69); Query OK, 1 row affected (0.26 sec) mysql> Create table student(id int, Name Varchar(10), ); Query OK, 0 ...
Read MoreHow changes, made in the current transaction, can be permanently recordednin MySQL database?
We can use COMMIT command to make the changes, made in a current transaction, permanently recorded in MySQL database. Suppose if we run some DML statements and it updates some data objects, then COMMIT command will record these updates permanently in the database.Examplemysql> START TRANSACTION; Query OK, 0 rows affected (0.00 sec) mysql> INSERT INTO Marks Values(1, 'Aarav', 'Maths', 50); Query OK, 1 row affected (0.00 sec) mysql> INSERT INTO Marks Values(2, 'Harshit', 'Maths', 55); Query OK, 1 row affected (0.00 sec) mysql> COMMIT; Query OK, 0 rows affected (0.06 sec)In this example, the COMMIT statement will ...
Read MoreWhat happens if I use both G and semicolon (;) termination symbol with a single MySQL statement?
As we know that \G option sends the command to MySQL server for execution and with the help of Semicolon (;) MySQL determines the end of the statement. It is also known that both of them have a different format of the result set.Now, if we will use both of those in MySQL statement then the output would be produced on the basis that which of them is encountered first by MySQL. For others, MySQL will produce an error. It can be understood with the help of the following example −mysql> Select CURDATE();\G +------------+ | CURDATE() | +------------+ | 2017-11-06 ...
Read MoreHow can I combine built-in-commands (g and G), used for executing a MySQL statement, with termination symbol semicolon (;) to get output without any error?
As we know that built-in-commands (\G and \g) send the command to MySQL server for execution and with the help of Semicolon (;) MySQL determines the end of the statement. It is also known that both of them have different format of the result set. For combining them and getting the result without error, we need to write two queries, one query with either \G or \g and other with a semicolon (;) at the end, in a single statement.ExampleCombining \G and Semicolon (;) −mysql> Select * from student\G select * from ratelist; *************************** 1. row *************************** Name: Gaurav ...
Read MoreIn MySQL, how we can get the total value by category in one output row?
With the help of the MySQL SUM() function, we can get the total value by category in one output row. For example in table ‘ratelist’ if we want to get the total value of category ‘price’ then we can use SUM() on price as follows −mysql> select SUM(price) as totalprice from ratelist; +------------+ | totalprice | +------------+ | 3237 | +------------+ 1 row in set (0.00 sec)The query above returns the total value of price in one output row.
Read MoreHow MySQL manage the behavior of a transaction?
MySQL can manage the behavior of a transaction with the help of the following two modes −Autocommit OnIt is the default mode. In this mode, each MySQL statement (within a transaction or not) is considered as a complete transaction and committed by default when it finishes. It can be started by setting the session variable AUTOCOMMIT to 1 as follows −SET AUTOCOMMIT = 1 mysql> SET AUTOCOMMIT = 1; Query OK, 0 rows affected (0.07 sec)Autocommit OffIt is not the default mode. In this mode, the subsequent series of MySQL statements act like a transaction, and no activities are committed ...
Read MoreWhile using the ROLLUP modifier, is it possible to use a MySQL ORDER BY clause to sort the result?
Actually ROLLUP and ORDER BY are mutually exclusive in MySQL hence it is not a good practice to use both of them in a query. But still, if we use ROLLUP in ORDER BY then the main disadvantage is that the summary rows would get sorted along with the rows they are calculated. It is also significant to notice that the sort order will decide the position of summary rows.The summary rows would be at the beginning of ascending order and at the end of descending order. Consider the following example to understand it more clearly −mysql> Select * from ...
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