Basic syntax of MySQL DDL and DML and DQL

Catalog

MySql Medium SQL Sentence classification

DDL Statement operation database

DDL Statement operation on Library

DDL Statement on the table in the Library

DML Basic syntax of statements

Data increase （INSERT sentence ）

Data modification （UPDATE sentence ） 

Data deletion （DELETE sentence ）

delete truncate drop difference

DQL Basic syntax of basic sentences （ Single table ）

DQL Basic syntax of advanced statements （ Multiple tables ）

Subquery  

where Type subquery

from Type subquery  

exists Type subquery

  Process control function statement （ Special inquiries ）

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MySql Medium SQL Sentence classification

SQL sentence , Structured query language (Structured Query Language), Is a special purpose programming language , It's one A database query and programming language , For data access and query 、 Update and manage relational database system , It's also a database Script file extension .

DDL Statement operation database

DDL Statement operation on Library

• Create database

•   View all databases

         • Be careful not to omit “s”

•   Delete database

•   Choose which database to use

DDL Statement on the table in the Library

• View the tables in the library

• Create table  

for example ： 

 

•   View the data table structure created

•   Modify the name of the table

• Modify the fields in the table

• Delete the fields in the table

• Add fields in the table

• Delete table

 

DML Basic syntax of statements

Data increase （INSERT sentence ）

         keyword INSERT, The main function is to add records to the database table .

•   grammar ：

  for example ：

•   Select a field to insert data

•   Insert data in full fields

         Be careful ： When a full field is inserted , If the primary key increases automatically , To set the default , It can be used default / null

•    Insert multiple pieces of data at once

       Unusual new methods

Data modification （UPDATE sentence ） 

         Keywords are UPDATE, The main function is to modify the existing records in the database table . It can be modified according to the conditions .

• grammar

for example ：

       Be careful ： Modification must have where Clause condition , No addition where Condition refers to modifying all

Data deletion （DELETE sentence ）

         Keywords are DELETE, The main function room deletes the existing records in the database table . You can delete according to the conditions .

• grammar

  for example ：

          Be careful ： Delete must have where Clause condition , No addition where Condition refers to deleting all

• truncate Clear the table  

 

delete truncate drop difference

• delete： Delete only the data in the table , Keep the index , Will not change the table structure
• truncate： Delete data from table + Indexes （ Reset index ）, Will not change the table structure
• drop： Delete all in the table and delete the table structure （ The watch is gone ）

DQL Basic syntax of basic sentences （ Single table ）

         Keywords are SELECT, The main function is to query the existing records in the database table .

• grammar

  for example ： 

• Table alias and field alias

1. Mode one ： Add after field name as+ Alias
2. Mode two ： Add an alias after the field name and wrap it in quotation marks
3. Mode three ： Add an alias directly after the field name without quotation marks

• Remove duplication distinct 

  Get rid of SELECT Duplicate records in the returned record result of query （ All return columns have the same value ）, Just return one

• WHERE Conditional clause

1. WHERE Conditional clauses are not required ;
2. WHERE Clause , You can add conditions to the query ;
3. Conditions ： Filter by , If not specified, modify all data of the table .

•  LIKE keyword （ Fuzzy query ）

•  IN keyword （ Range queries ）

1. The value of the queried field , At least with IN One value in the parentheses after is the same ;
2. Multiple values are separated by English commas .

•  NULL Value query

1. NULL representative “ No value ”;
2. Different from zero 0 Null string ;
3. Can only appear when the definition allows NULL Field of ;
4. To be used IS NULL or IS NOT NULL Compare operators to compare .

 DQL Basic syntax of advanced statements （ Multiple tables ）

 

• Equivalent query ( Inline query ) And non equivalent query

1. Similar to single table query , All are SELECT sentence ;
2. Put multiple tables in FROM after , And separated by commas ;
3. You can use AS Keywords are aliases , Easy to quote ;
4. If there is no duplicate name query field, the designation of the data table can be omitted .

 

• Inline query （inner join）

grammar ：SELECT * FROM surface 1INNER JOIN surface 2 ON Conditions

INNER JOIN： When at least one of the tables matches , Then return the record .

 INNER JOIN give an example ： 

 

•   Outreach query （left join/right join）

grammar ：SELECT * FROM surface 1 LEFT| RIGHTJOIN  surface 2 ON strip

LEFT JOIN： From the left table （ surface 1） Return all records in , Even on the right （ surface 2） There are no matching lines in .

RIGHT JOIN： From the right table （table_2） Return all records in , Even if On the left （table_1） There are no matching lines in . 

As shown in the following figure ：

 

 LEFT JOIN give an example ：

 RIGHT JOIN give an example ：

 

• Find the union of two queries (UNION) 

union Is to find the union of two queries .

union What is merged is the result set , Do not distinguish from which table , So you can merge the data queried from multiple tables .

Be careful ：

1. When the column names are inconsistent , The header of the first table will prevail , And its columns .
2. union Duplicate rows will be filtered out .
3. If the number of columns in the queried table is not equal , Will report a mistake .
4. Sorting in each clause is meaningless ,mysql It will be ignored when merging .
5. If the sort and in the clause limit It makes sense to combine .
6. You can sort the consolidated whole table .
7. union all Is to find the union of two queries , But it won't filter out the repetition , It's all displayed

 

Subquery  

Subquery , It's also called internal query , All subqueries must be enclosed in parentheses

 

where Type subquery

 

from Type subquery  

• Take the query results of the inner layer as a temporary table , For the outer layer sql Query again .
• The query result set can be treated as a table .
• The temporary table should use an alias .

 

 exists Type subquery

• Put the outer layer sql Result , Get the inner layer sql To test , If the inner layer sql establish , Then the line takes out .
• The inner query is exists After the query .

 

  Process control function statement （ Special inquiries ）

 

•  IF() function

•    IFNULL() function

 

• Simple case

 

• Search for case