MySQL Expressions

Expressions are a common element of SQL statements( statements are select, delete and update).
they often occur in the where clause of select, delete & update.

For Example:
DELETE FROM users WHERE id=5;

Here id=5 are expressions.

Following are the different components of SQL Expression:

•  Numeric Expressions
  Exact value Literal are: 11,12.33, 44.333
  Approximate Literal are 1233E2, 44333E3
  Operations on decimal value have precision upto 64 digits, currently its 30 character.
  In Approximate, mantissa allows upto 53 bit of precision which is about 15 decimal digits.
•  String Expressions
  String like 'sql' or mysql
• Temporal Expressions
  It include dates, times and datetime values like Select ‘2012-01-01’ + interval 10 day

• Functions like current(), version(), avg(), sum()
• Regular Express (Regex)

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MySQL Delete Table Example

Create a table

CREATE TABLE IF NOT EXISTS `users` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(200) DEFAULT NULL,
  `image` varchar(255) DEFAULT NULL,
  `company` varchar(150) DEFAULT NULL,
  `created` datetime DEFAULT NULL,
  `modified` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=1;

Delete record(s) from table

DELETE FROM users WHERE id=5;

It will delete a record where id is 5

Delete record from two table when they are in join.

DELETE users1,users2 FROM users1,users2,users3 WHERE tusers1.id=users2.id AND users2.id=users3.id

Keep in Mind following points..

• If we specify the LOW_PRIORITY keyword in query, execution of the DELETE is delayed until no other clients are reading from the table.
• For MyISAM tables, if we specify the QUICK keyword in query, then the storage engine will not merge index leaves during delete, which  will speed up certain kind of delete.
• If we specify the IGNORE causes MySQL to ignore all errors during the process of deleting rows. 

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