Tags: tab exist false ACL any did COM java.net outputProblem:Statement 1: SELECT * FROM table1 A where a.col1 not in (Select col1 from Table2 B) Reprint Annotated Source: http://x-spirit.iteye.com/, http://www.blogjava.net/zhangwei217245/If so, there should have been a piece of data, the result is not.If I rewrite it like this:Statement 2: SELECT * FROM table1 A where isn't exists (SELECT * from table2 B where b.col1 = a.col1) The result is correct an
Where exists 2The basic select usage was previously described in terms of personal understanding. Of course, exists is more than just a select. For example, update can also use where existsTo continue the previous explanation, I saw from the internet that said. where exists and in efficiency are not the same, let's do a test to compare how different.First create
Tags: oop word query. NET view loop pop fonts left connectionIn the actual development, we often need to compare two or more table data differences, compare those data the same data is not the same, then we have three ways to use: 1. In or not in,2. exist or notexist,3. Use a connection query (inner Join,left join or right join). Looking at the data below, we are ready to select the Depart_ information that does not exist in the Depart_info pid in User_info. There are table 1:depart_info Table
Tags: more than tab Release 2 VARCHAR2 order case OTT subquery spanOfficial documents:home/database/oracle Database Online Documentation 11g Release 2 (11.2)/Database administrationEXISTS ConditionAn EXISTS condition tests for existence of rows in a subquery.The EXISTS condition tests whether rows exist in the subquery.EXISTS (SQL returns the result set as true)Not EXIS
1. Determine if the data table existsMethod One:Use Yourdb;GoIf OBJECT_ID (n ' tablename ', n ' U ') is not nullprint ' exists 'Elseprint ' does not exist 'For example:Use Fireweb;GoIf OBJECT_ID (n ' temp_tbl ', n ' U ') is not nullprint ' exists 'Elseprint ' does not exist 'Method Two:Use [instance name]GOIF EXISTS (SELECT * FROM dbo. SysObjects WHERE id = objec
not EXISTS (SELECT * from Testa as b where a.cid = B.cid and A.buytime Here are a few key methods, exists, group by and having; According to a single understanding: exists to determine whether the expression inside the parentheses returns the result is true, return true will be the current main table data out, otherwise discard the result (Bo Master wrote the a
The following objects are judged for existence: Database, table, proc, trigger, temp table, index.1. Judge the database:if exists (SELECT * from sys.databases WHERE name = ' database name ')drop database [DB name]2. Judgment table:SQL codeif exists (select * from sysobjects where id = object_id (n ' [table name] ') and OBJECTPROPERTY (ID, n ' isusertable ') = 1)drop table [table name]3. Determine the stored
This article mainly introduces node. fs. exists method usage instructions. This article introduces fs. exists method description, syntax, receive parameters, use instances, and implement Source Code. For more information, see
Method description:
Test whether a file in a path exists.
The callback function contains an exists
-- Determine whether ifexists (select1frommaster .. sysdatabaseswherenameTestDB) printTestDB elseprintTestDB does not exist -- determine whether the table has ifexists (select * fromTestDB .. syscolumnswhereidobject_id (TestDB. dbo. testTb) in the print table TestTb, else exists.
-- Determine whether if exists (select 1 from master .. sysdatabases where name = TestDB) print TestDB else print TestDB does no
Mysql in and EXISTS optimization example introduction, mysqlexists
Optimization Principle: small tables drive large tables, that is, small datasets drive large datasets.
############ Principle (RBO )#####################
Select * from A where id in (select id from B) is equivalent to: for select id from Bfor select * from A where A. id = B. id
When the dataset of Table B must be smaller than the dataset of Table A, in is better than
In the previous article "Supplement to SQL query optimization -- (in)", we mainly discussed the query statements such as select... from... where... in. Next, let's discuss Select... from... where... exists. The query speed of exists is much higher than that of in, which is well known, but we cannot say that exists is used to replace in completely. Why? Let's take
Php checks whether a variable constant exists. In php, we have common variables and constants for variable types. next I will introduce how to judge whether constants and variables are stored in php, for more information about variable types in php, see common variables and constants. next I will introduce how to judge whether constants and variables are stored in php, for more information, see.
The defined () function checks whether a constant
statement. It is the sorting of the operation results of the entire set. The column name, alias, or specified by position cannot be used for columns not displayed in the first select statement.
⑥ The first select statement receives the column name, alias, or location mark. The column name of the query result is consistent with that of the first select statement.
7. It can be used for subqueries.
2 Use Cases of exists and in
You can directly write mul
Let's take a look at the following example: Two system tables in Oracle. emp, dept. example:
1: not exists (not in)
Not exists:
This statement returns the result set that the select * from scott. dept d where e. deptno = d. deptno and d. deptno = 10 conditions meet. That is,
The returned result set does not have a record of d. deptno = 10, that is, the emp table does not have a record of d. deptno = 10 in t
Use shell script to monitor whether a process exists without a startup instance, first on the code dry:
#!/bin/sh
ps-fe|grep processstring |grep-v grep
if [$-ne 0]
then
echo "START process ..."
else
#####Processstring represents a process feature string that can be queried to a unique process's character string0 indicates the existence of$? -ne 0 does not exist, $? -eq 0 Exists
Timed execution
Label:There are two simple examples to illustrate the efficiency issues of "exists" and "in" 1) SELECT * from T1 where exists(select 1 from T2 where t1.a=t2.a); When the T1 data volume is small and the T2 data is very large, the query efficiency of T1 2) SELECT * from T1 where t1.a in (select t2.a from T2); T1 data volume is very large and T2 data amount of hours, T1>>T2, 2) query efficiency is high.
are as follows: At this point, the result of the query and the original user table to make a Cartesian product, the result is as follows: At this point, the results are filtered according to our user.id in order.user_id (comparing the values of the ID column and the user_id column for equality and will not be equal to the deletion). Finally, two qualifying data are obtained. ExistsSpecifies a subquery that detects the presence of a row. Iterate over the outer surface and see if the rec
--Determine if the library exists, or delete itIf exists (select name from master: sysdatabases where name in (' Db_name '))Drop Database Db_nameGo--Determine if the stored procedure exists, or deleteif exists (select * from sysobjects where objectproperty (object_id (' Proc_name '), ' isprocedure ') =1)drop procedure
This article mainly introduces how to use function_exists to check whether a function exists in php. The example shows how to use function_exists to check whether a function exists and call related techniques in php. It is very useful and requires
This article mainly introduces how to use function_exists to check whether a function exists in php. The example sho
Js determines whether an element exists. This function is very practical in the project. I have collected and sorted out some common skills. For details, refer
1. Determine whether a form element exists (1)
The Code is as follows:
Code highlighting produced by Actipro CodeHighlighter (freeware) http://www.CodeHighlighter.com/--> if ("periodPerMonth" in document. theForm){Return true;}Else {Return false;}
The content source of this page is from Internet, which doesn't represent Alibaba Cloud's opinion;
products and services mentioned on that page don't have any relationship with Alibaba Cloud. If the
content of the page makes you feel confusing, please write us an email, we will handle the problem
within 5 days after receiving your email.
If you find any instances of plagiarism from the community, please send an email to:
info-contact@alibabacloud.com
and provide relevant evidence. A staff member will contact you within 5 working days.