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SQL common syntax records
2022-07-05 21:22:00 【coder chen】
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 :
- FROM Clause assembly data ( Including passage ON Connect );
- WHERE Clause ;
- GROUP BY grouping ;
- Using aggregate functions to calculate ;
- HAVING Filter groups ;
- Calculate all expressions ;
- SELECT Field of ;
- ORDER BY Sort ;
- 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 .
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