SQL quick query

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1、 explain ： Create database CREATE DATABASE database-name2、 explain ： Delete database drop database dbname3、 explain ： Backup sql server--- establish Backup data deviceUSE masterEXEC sp_addumpdevice 'disk', 'testBack', 'c:\mssql7backup\MyNwind_1.dat'--- Start Backup BACKUP DATABASE pubs T

• relational database , Based on the relational model , Use relationship （ surface ） Store the data , At the same time, the integrity constraint is defined . Common relational database systems include ：Oracle、MySQL/MariaDB、SQL Server、PostgreSQL wait .
• SQL, Structured query language , Standard language for accessing and manipulating relational databases .SQL It's declarative , Is a set oriented programming language .

1、 Single table query

SELECT col1, col2 AS c2 --  Column alias  FROM t; --  The basic query SELECT * FROM t; --  Query all fields SELECT col1, col2, … FROM t WHERE conditions; --  Filter conditions SELECT col1, col2, … FROM t ORDER BY col1 ASC, col2 DESC; --  Sort SELECT col1, col2, … FROM t ORDER BY col1 ASC, col2 DESC OFFSET m ROWS FETCH FIRST n ROWS ONLY; --  Limit the number of  LIMIT n OFFSET m; --  Non standard implementation SELECT col1, col2, agg_fun() --  Aggregate functions  FROM t GROUP BY col1, col2 --  Group summary  HAVING conditions; --  Filter after grouping 

2、 Multiple table joins

SELECT t1.col1, t2.col2, … FROM t1 INNER JOIN t2 ON conditions; --  Internal connection  SELECT t1.col1, t2.col2, … FROM t1 LEFT JOIN t2 ON conditions; --  Left connection SELECT t1.col1, t2.col2, … FROM t1 RIGHT JOIN t2 ON conditions; --  The right connection SELECT t1.col1, t2.col2, … FROM t1 FULL JOIN t2 ON conditions; --  Full connection SELECT t1.col1, t2.col2, … FROM t1 CROSS JOIN t2 ON conditions;--  Cross connect SELECT a.col1, b.col2, … FROM t1 a --  Table alias  JOIN t1 b ON conditions; --  Self join 

3、 Set operations

SELECT col1, col2, … FROM t1 UNION [ALL]SELECT c1, c2, … FROM t2; -.........