SQL common syntax records

with … as … select

• Writing SQL when , For two already order by Statement of union When the failure , Later it was found in Oracle sql in , requirement order by yes select The last statement of the statement , And one select Only one... Is allowed in the statement order by sentence , and order by Must be located throughout select At the end of the sentence .
• union The operation actually does two parts ： Result set merge + Sort ,
• union all Simply merge result sets , Do not sort , Efficient than union high
• Therefore use with … as … select

with 
	s1 as (select  * from table1 t1  order by t1. c1 ),
　　 s2 as ( select  * from table1 t2  order by  t2.c1 )
 	select  *  from s1 union all ( Here can be replaced by  union ) select * from s2

Refer to this ：https://www.icode9.com/content-2-602860.html

• Yes with … as …select Explanation ：

It's actually a lot of things that are used repeatedly sql Put the sentence in with as Inside , Take an alias , Later queries can use it , In this way, for large quantities of sql Statement plays an optimization role , And it's clear

with as Equivalent to virtual view

Refer to this ：https://www.cnblogs.com/mingforyou/p/8295239.html

case when

• CASE WHEN Conditional expression function ： similar JAVA Medium IF ELSE sentence

CASE WHEN SCORE = 'A' THEN ' optimal '
     WHEN SCORE = 'B' THEN ' good '
     WHEN SCORE = 'C' THEN ' in ' ELSE ' fail, ' END

Refer to this article ：https://blog.csdn.net/rongtaoup/article/details/82183743

group by ,where,having

group by ,where and having Can be used at the same time , But should pay attention to SQL Execution order of ,where>group by>having>select, Can't be in where Queries only after adding groups

https://deepinout.com/sql/sql-dql/sql-select-group-by.html

exists

sql recursive ？

sql Execution order

A complete sql The order of execution is ：

1. FROM Clause assembly data （ Including passage ON Connect ）;
2. WHERE Clause ;
3. GROUP BY grouping ;
4. Using aggregate functions to calculate ;
5. HAVING Filter groups ;
6. Calculate all expressions ;
7. SELECT Field of ;
8. ORDER BY Sort ;
9. LIMIT Screening

sql The two orders of ：

Writing order

select … from … where … group by … having … order by …

Execution order

from>where>group by>having>select Field of >distinct>order by>limit

sql Perform principle analysis ：https://deepinout.com/sql/sql-dql/sql-select-statement-execution-order.html

distinct explain

DISTINCT Need to precede all column names , If written SELECT name, DISTINCT attack_range FROM heros Will report a mistake .

DISTINCT In fact, it is to de duplicate the combination of all the following column names

not in Failure problem oracle

reason ：not in There is null, Need to put null Get rid of

DBLink : It is mainly used for cross database connection query

https://help.aliyun.com/document_detail/99941.html

Use ：

establish DBLink

CREATE PUBLIC DATABASE link dblink CONNECT TO user name IDENTIFIED BY password USING ‘ service ip/serviceName’;

Inquire about Current library Of dblink

select * from dba_db_links;

Delete the dblink

DROP PUBLIC DATABASE LINK name （dblink name ）;

Then pay attention to the cross database joint table Add @dblink name

update [email protected] A set A.user_ORGID =(select B.ORGID from ORG_EMPLOYEE B where B.EMPID=A.EMPID)

Condition query null value judgment

where Keyword followed by manager is not null perhaps manager is null

order by Chinese sort and null sort

https://deepinout.com/sql/sql-dql/sql-select-order-by.html

Common functions

nvl()

It is a null conversion function ,nvl( expression 1, expression 2), If the expression 1 The value of is empty ,nvl The return value is the expression 2 Value , Otherwise return the expression 1 Value .

The value of an expression can be numeric , Character type and date type ,DNA The data type must be consistent .