MySQL system functions and federated queries

A function is a keyword in SQL that is used to manipulate the data in a field. A function is a command, usually used in conjunction with a field name or an expression, to process the input data and produce a result

Common functions

Control functions	String functions	Mathematical functions	Date-time functions	Summary functions
Case	SUBSTRING	Abs	DATEDIFF	Avg
IF	Left	Floor	Date_add	COUNT
Nullif	LENGTH	RAND	Str_to_date	MAX
Ifnull	LOWER		Date_format	MIN
	REPLACE			SUM
	Concat

1. Left connection

The left table is the main query data

2. Right connection

The right-hand table is the main query data

3. Internal connection

A typical connection operation, using a comparison operator like = or <>. Includes both equal and natural connections.

An inner join uses a comparison operator to match rows in two tables based on the values of the columns that are common to each table.

Left connection

The result set for the left join includes all rows from the left table specified in the IEFT clause, not just the rows that match the joined columns. If a row in the left table does not have a matching row in the right table, all select list columns in the right table in the associated result set row are null values.

Left JOIN

Select A.* , B.* from A  leftjoinon= B.colname

Right connection

The right connection is the reverse connection of the left connection. All rows of the right table will be returned. If a row in the right table does not have a matching row in the left table, a null value will be returned for left table.

Right JOIN

Grammar Select A.* , B.*from  A  right  join on = B.colname

Internal connection

Displaying the data associated with two tables is equivalent to asking for the intersection of two sets.

INNER JOIN

Grammar Select A.* , B.* from A  innerjoinon= B.colname

Union [All]

The Union operator can combine the results of two or more SELECT statements, with no duplicates. In other words, if the output of a row exists in a query result, then the records of the same row of the other query results will no longer be output. When using the union operator, you must select the same number of fields, the same number of field expressions, the same data type, the same order (but not the same length) in each SELECT statement

If you need not to remove duplicates, add the ALL keyword

MySQL system functions and federated queries