19、 MySQL -- SQL statements and queries

        SQL Structured query statements （Structure Query Language） Include DDL Data definition language （Data Definition Language）、DML Data operation language （Data Manipulation Language） and DCL Data control language （Data Control Language）.

        DDL Include create、frop、alter、truncate sentence

        DML Including increase insert Delete delete truncate Change replace updata check select

        DCL Include grant revoke

         This article discusses SELECT Statement and write SQL Notes on sentences

1.select sentence

        select @@autoincrement Find variables

        SELECT  *|{[DISTINCT]  column|select_expr  [alias], ...]}    [FROM [database.]table]

{} Braces indicate that the content inside must be written with

[] Brackets indicate that the contents inside are optional

| One out of two

, … It means that the previous content can be repeated several

Lowercase letters indicate that the actual name should be used here

* Represents any field

DISTINCT Indicates that the result of the query is de duplicated

[alias] Alias

# Query all fields

# Use the alias

 2.where Clause

        where condition Need to cooperate with select Statements use condition On condition that , Also known as filter condition

        # Can't be in WHERE Use column aliases in Clause

3.order by Clause

        [ORDER  BY  {column, expr, numeric_position} [ASC|DESC]];

        Need and select Statements use ASC Ascending ,DESC Descending , Default ascending order 、

4. Group functions

        Group function is also called aggregate function , It operates on a collection of rows , Give a result for each group

        Common group functions

        AVG([DISTINCT] expr) ： averaging

        COUNT({*|[DISTINCT] } expr) ： Count the number of lines

        MAX([DISTINCT] expr)  ： For maximum

        MIN([DISTINCT] expr)  ： For the minimum

        SUM([DISTINCT] expr)  ： Sum up

        # Nested use is not allowed between group functions , It can be a function expression , Multiple aggregate functions can appear

        GROUP_CONCAT() function

5.HAVING Clause

        [HAVING where_condition]  It is specially used to filter the grouped results

        HAVING Group functions can be used in Clauses ,where Cannot be used with group functions

6. Multi table join query

        SELECT  paymentno, pen.playerno, amount,NAME  

        FROM  penalties pen, players p  

        WHERE  pen.playerno = p.playerno;

7. Subquery