SQL related knowledge - DQL

1.DQL: Look up the records in the table

select * from Table name

grammar ：

                select

                         Field list

                from

                         List of table names

                where

                         List of conditions

                group by

                        Grouping field

                having

                        Conditions after components

                order by

                        Sort

                limit

                        Paging limit

2. Basic query

        1. Multiple field queries

                select Field name 1, Field name 2...from Table name ;

                * Be careful ：

                        If you query all fields , You can use * Instead of the field list

        2. Remove duplication

                        distinct

        3. Calculated column

                        1. Generally, four operations can be used to calculate the values of some columns .( Generally, only numerical calculation will be carried out ).

                        2.ifnull( expression 1, expression 2):null Operations involved , The results are all null

                        expression 1： Which field needs to determine whether it is null, If the field is null, Just use the expression 2 Replace .

        4. names

                        as：as You can omit it , Use spaces .

3. Conditions of the query

1.where Clause followed by condition

2. Operator

        >,<,<=,>=,=,<>（ It's not equal to ）

        BETWEEN...AND...

        IN( aggregate )

        LIKE： Fuzzy query

                1._: Any single character

                2.%： More than one arbitrary character

        IS NULL

        AND perhaps &&

        OR perhaps ||

        not or |

4. Sequential query

grammar ：order by Clause

                order by Sort field 1 sort order 1, Sort field 2 sort order 2....

sort order ：

                ASC: Ascending , default .

                DESC: Descending .

Be careful ：

        If there are multiple sorting conditions , The condition value of the current edge is the same , To judge the second condition

5. Aggregate functions

Aggregate functions ： Take a column of data as a whole , Do the longitudinal calculation

        1.count： Calculate the number of

                1. Generally, non empty columns are selected ： Primary key

                2.count(*)

        2.max: Calculate the maximum

        3.min: Calculate the minimum

        4.sum： The calculation and

        5.avg： Calculate average

        * Be careful ： The calculation of aggregate functions , exclude NULL value .

                Solution ：

                        1. Select columns that do not contain non null to calculate

                        2.IFNULL function

6. Group query

1. grammar ：group by Grouping field ;

2. Be careful ：

        1. Fields to be queried after grouping ： Grouping field , Aggregate functions

        2.where and having The difference between ：

                1.where Limit before grouping , If you don't do that , They don't participate in the grouping .having Limit after grouping , If the result is not satisfied , It won't be found out

                2.where You can't follow an aggregate function ,having We can judge the aggregate function .

7. Paging query

1. grammar ：limit Index started , Number of queries per page ;

2. The formula ： Index started = （ The current number of pages - 1）* Number of entries per page

Sample code ：

Three pieces of data per page ：

        select * from students limit 0,3;-- first page 

        select * from students limit 3,3;-- The second page 

        select * from students limit 6,3;-- The third page 

3.limit It's a MySQL Peculiar .