SQL language

SQL Language

elements of grammar

notes

#  Use '#' notes --  Use '--' notes /* Multiline comment */

Create use database

CREATE DATABASE db01;USE db01;

Create table

CREATE TABLE table01 (  id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,  col1 INT NOT NULL DEFAULT 1,  col2 VARCHAR(45) NULL,  col3 DATE NULL);

Modify table

#  Add new column ALTER TABLE table01 ADD col CHAR(20);#  Modify the column  ALTER TABLE  Table name  CHANGE  Original field name   new field name   Field type   constraint condition ALTER TABLE table01 CHANGE col col4 CHAR(32) NOT NULL DEFAULT 'xxx';#  Delete column ALTER TABLE table01 DROP COLUMN col4;#  Delete table DROP TABLE table01;

Additions and deletions

#  Add data INSERT INTO table01(col1, col2)VALUES(val1, val2);#  Table data migration CREATE TABLE newtable ASSELECT * FROM table01;#  to update UPDATE table01SET col = valWHERE  Conditions ;#  Delete DELETE FROM table01WHERE  Conditions ;

Inquire about

Basics

#  duplicate removal SELECT DISTINCT * FROM table01;#  Limit return SELECT * FROM table01 LIMIT 5, 2; #  from 5 Start （ barring 5） Return two rows of data , namely 6,7#  Sort  ASC Ascending ,DESC Descending SELECT * FROM table01ORDER BY col1 DESC, col2 ASC;#  Screening SELECT * FROM table01 WHERE  Conditions ;

Calculated field

Letter Count	say bright
AVG()	Returns the average of a column
COUNT()	Returns the number of rows in a column
MAX()	Returns the maximum value of a column
MIN()	Returns the minimum value of a column
SUM()	Returns the sum of values in a column

#  give an example ： Calculate the weight removed col1 The mean value of the column is named avg_colSELECT AVG(DISTINCT col1) AS avg_colFROM table01;

Text processing

have access to LIKE + Regular expressions Filter text

SELECT *FROM table01WHERE col1 LIKE '[^AB]%'; --  Don't to  A  and  B  Any text at the beginning 

You can also use some functions for text processing ：

function	explain
LEFT()	The character on the left
RIGHT()	The character on the right
LOWER()	Convert to lowercase characters
UPPER()	Convert to uppercase characters
LTRIM()	Remove the space on the left
RTRIM()	Remove the space on the right
LENGTH()	length
SOUNDEX()	Convert a string into an alphanumeric pattern that describes its phonetic representation

grouping

Grouping is to put rows with the same data value in the same group . You can use the summary function to process the same grouped data .
WHERE Filter line ,HAVING Filter grouping , Row filtering should precede group filtering .

SELECT col1, COUNT(*) AS numFROM table01WHERE col1 > 2GROUP BY col1HAVING num >= 2;

We also need to pay attention to the following points

• GROUP BY Clause appears in WHERE After Clause ,ORDER BY Before clause ;

• In addition to summary fields ,SELECT Every field in the statement must be in GROUP BY Given in Clause ;

• NULL Your guild is divided into separate groups ;

• majority SQL Implementation does not support GROUP BY Columns have variable length data types .

Subquery

Only one field of data can be returned in the subquery . The results of subqueries can be used as WHRER Filter condition of statement :

SELECT *FROM table01WHERE col1 IN (SELECT col1               FROM table02);

Link query

Join is used to join multiple tables , Use JOIN keyword , And conditional statements use ON instead of WHERE.
Equivalent connection ：

SELECT A.value, B.valueFROM tablea AS A INNER JOIN tableb AS BON A.key = B.key;

It can also be used indefinitely INNER JOIN, Instead, use normal queries and in WHERE Join the columns to be joined in the two tables by equivalent method .

SELECT A.value, B.valueFROM tablea AS A, tableb AS BWHERE A.key = B.key;

Natural join ：
Natural join is to join columns with the same name through equivalence test , Columns with the same name can have more than one .

SELECT A.value, B.valueFROM tablea AS A NATURAL JOIN tableb AS B;

External connection ：
It is divided into left outer connection , Right outer connection and all outer connection , The left outer join is to keep the unrelated rows in the left table , The right outer connection is the same .

SELECT A.value, B.valueFROM tablea AS A LEFT OUTER JOIN tableb as bON A.col1 = B.col1;

Combination query

Use UNION To combine two queries , If the first query returns M That's ok , The second query returns N That's ok , The result of the combined query is generally M+N That's ok . Each query must contain the same columns 、 Expressions and aggregation functions .
By default, the same line will be removed , If you need to keep the same company , Use UNION ALL.

SELECT *FROM table01WHERE col1 = 1UNIONSELECT *FROM table01WHERE col1 =2;

Use view query

View is a virtual table , It doesn't contain data itself , It can't be indexed . The operation on the view is the same as that on the normal table .
Here is a simple view creation ：

CREATE VIEW myview ASSELECT  sentence ;

Rights management

stay mysql in , The account information is saved in mysql In this database

USE mysql;SELECT * FROM user;

Account operation

#  Create an account CREATE USER myuser IDENTIFIED BY ' password ';#  Change user name RENAME myuser TO newname;#  Delete user DROP USER newname;#  View permissions SHOW GRANTS FOR myuser;#  to grant authorization GRANT SELECT, INSERT ON mydatabase.* TO myuser;#  Delete permission REVOKE SELECT, INSERT ON mydatabase.* FROM myuser;#  Change password SET PASSWROD FOR myuser = Password('new_password');

author ：S++

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