<> Process control statement
<> similar java in if else sentence
<> similar java in switch case sentence
<> Aggregate function （ Cannot be nested MIN(AVG(salary)), But in oracle Li support ）
Aggregate functions act on a set of data , And returns a value for a set of data . AVG(),SUM(),MAX(),MIN(),COUNT()
<>GROUP BY(SELECT The query field must be group by Field of , conversely ,group
by Fields declared in may not appear in SELECT in , However, if there is an aggregate function in the query field, it is OK )
<>HAVING Use of （ If the filter condition has an aggregate function , Put on where There will be problems in the , So it can only be placed in having in ）
<>SQL grammar
<>SQL Statement execution
* First step ： implement FROM
* Step 2 ：WHERE Conditional filtering
* Step 3 ：GROUP BY grouping
* Step 4 ： implement SELECT Projection column
* Step 5 ：HAVING Conditional filtering
* Step 6 ： implement ORDER BY sort
<> Subquery
<> Related sub query
<>EXISTS And NOT EXIST keyword
<>MYSQL data type
<> Integer type
<> Date type
<> String type
<>char and varchar difference
<>text type
<>ENUM type
<>Set type
<> Binary type
<> Data type selection summary
<> constraint
<> Non NULL constraint
Set field not to when creating table null
<> Unique constraint （unique）
If a unique constraint is added to a field , This means that this value is unique in this table , A second identical value does not appear , But if you add to this field more than once null Time , It will not be limited , It will be inserted .
<> Compound unique constraint
<> Delete uniqueness constraint
<> Primary key constraint （ infer other things from one fact , There are compound unique constraints , There must also be a composite primary key constraint ）
A table has only one primary key , The feature is non empty and unique
<> Foreign key constraint （FOREIGN KEY）
<> features
* A table can have multiple foreign keys
* When deleting data, delete the sub table before deleting the main table
* When a foreign key constraint is added, the corresponding normal index is created
<> Constraint level
<> Use of constraint levels
<> Delete foreign key
<> Development scenario （ Foreign key constraints are not recommended , Too limited ）
<>SQL
<> What is a view ?
<> Create view
create VIEW View name as sql sentence
<> view a chart
Technology