MySql filters, polymeric grouping, join query

Filters

Comparison Operators

• Equal to: = (note not ==!)
• Does not mean:! = Or <>
• Greater than:>
• Greater than or equal:> =
• Less than: <
• Less than or equal: <=
• IS NULL
• IS NOT NULL

Logical Operators

• And: and
• 或：or
• Non-: not

Other operations

• Sort by: order by
  • Positive sequence: asc
  • Reverse: desc
  • 例：select * from students order by age desc ;
• Limit: limit
  • M data display control: limit m
  • The number of rows from the beginning of the subscript m is the n data show: limit m, n
  • 例：select * from students limit 3,2 ;
• Deduplication: distinct
  • 例：select distinct * from students;
• Fuzzy query: like
  • Any one character: _
  • Any number of characters:%
  • 例：select * from students where name like '%abc_' ;
• Range queries
  • Continuous range: BETWEEN a AND b is equivalent to a <= value <= b
  • Return interval: IN
  • 例：select * from students where id in(1,3,5,7);
• Alias: as

Aggregated packet

Common aggregate functions

• Statistics Number: COUNT (column)
• Maximum: MAX (column)
• Minimum: MIN (column)
• Sum: SUM (column)
• Average: AVG (column)
• All values ​​listed fields: GROUP_CONCAT (column)

Grouped query: group by

• In case of a packet, the packet field and a polymerization field can only occur
• Other field has no meaning, I will complain!
• Select fields from table group by field;
• Select field, count (*) from table group by field;

Polymerization Screening: having

• Plus having conditional expressions may be made to limit the output results
• Select fields from table group by 1 Field 1, Field 2 Field HAVING 2> = 80;

Execution order

• If a query contains an alias at the same time (as), aggregate functions, where, having, then their order of execution
  • First execution: where
  • Then execute: a function of polymerization and aliases
  • The last execution: having

Subqueries

The results of a query to stay for the next query (select nested select)

Claim:

• 1) nested inner query
• 2) must always appear in parentheses

Example:

• Find the average age of students
  • select avg(age) from students;
• Find out the data is greater than the average age of
  • select * from student where age > 19;
• The average age of SQL statements obtained is used to find the average age is greater than the statement
  • select * from students where age > (select avg(age) from students);

Join query

En: inner join

• The unconditional connection:
  • The unconditional connection, also known as cross-connect / connected Cartesian
  • Each turn every combination will first table and another table
  • select * from 表1 inner join 表2
• Conditional inner join:
  • On the basis of internal links on unconditional, plus a clause on
  • When connected, screened those records meaningful to combine
  • select * from 表1 as a inner join 表2 as b on a.id = b.id

Outer connecting: {left | right} join

• Left outer: (in the left table reference)
  • When connecting two tables do not match the join condition when
  • Data left in the left table, the data table to the right filling NULL
  • select * from 表1 as a left join 表2 as b on a.id = b.id
• Right outer join: (reference table to the right)
  • To make connections when the two tables, does not match the join condition when
  • Data left in the right table, the data table to the left to fill NULL
  • select * from 表1 as a right join 表2 as b on a.id = b.id