A brief discussion on MySQL drifting (7)

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1) Briefly introduce Select

select selection_list  //选择哪些列  
from table_list  //从何处选择行  
where primary_constraint  //行必须满足什么条件  
group by grouping_columns // 怎样对结果分组  
having secondary_constraint  //行必须满足的第二条件  
order by sorting_columns //怎样对结果排序  
limit count  //结果限定

Note:
All keywords used must be given exactly in the order above. For example, a having clause must follow the group by clause and before the order by clause;
Everything in the above syntax except the select keyword is optional. Some databases require the from keyword, but MySQL allows expressions to be evaluated without referencing any tables

2) Ordinary query

select * from table01;//查询当前表里所有的列的数据 
select name,age from table01;//查询当前表里name列和age列的所有数据 
select (2+3)/2.5;//表达式求值而不引用任何表

3) Conditional query

select * from table_new where age>20;

#4) Sorting query

Keyword: order by – This clause sorts the results returned by the query by one or more columns
asc means ascending order, which is the default value, desc means descending order
It should be noted that:
desc/asc keyword only applies to the column name immediately before it
Case: select * from table_new order by age desc;

5) Query grouping and row counting

The group by clause returns grouped query results according to the given column name, which can be used to query columns with the same value , if in addition to counting, the value of a column is returned, then the group by statement needs to be used, otherwise the records cannot be calculated and an error occurs;
Case: select name,dizhi,count(*) from table_new group by name,dizhi;

If you want to query how many records there are in a table?
You can use the following statement directly:
Syntax:

select count(*) from 表名； 
Case:  select count(*) from table_new;

6) Query multiple tables

Now you need to query the same name from table table_new and another table table_new2 Field, print it out;
Case: select t1.name,t2.name from table_new as t1,table_new2 as t2 where t1.name=t2.name;

Explanation: as is represented as this Table alias.

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