find duplicates query sql

code is actually executed ?". When debugging your application, a good feature of LINQ is that it can easily view what SQL code is actually executed. After Visual Studio 2008's beta2 version, you can use the new plug-in program, the LINQ to SQL viewer, to easily view (and test) Any LINQ to SQL Query expressions. You ca

Brief introduction In SQL Server, an index is an enhanced existence, which means that SQL Server can still implement functionality even without indexes. However, indexes can greatly improve query performance in most cases, especially in OLAP. To fully understand the concept of the index, you need to know a lot of the knowledge of the original rationality, includ

number =A. Employee number)Note: this should be done to find out the maximum value of each employee's order amount, along with the information of the employee's corresponding order.Many people do this:SELECT * from order form where amount in (select Max (amount) from order Form group by employee number)This practice is wrong.3.6 quantifier QueryCommonly used quantifiers any, all, and some, where any and some are synonyms, as long as the subquery has

example:Statement 7:where conditions are independent.Selecto.id,o.order_number,o.customer_id,c.id,c.nameFrom ORDERS O left Outerjoin CUSTOMERS C on c.id=o.customer_idWhereo. Order_numberStatement 8: Place the WHERE condition in statement 7 behind on.Selecto.id,o.order_number,o.customer_id,c.id,c.nameFrom ORDERS O left Outerjoin CUSTOMERS C on c.id=o.customer_id ANDO. Order_numberFrom the results of statement 7 and statement 8 queries, it is clear that the result of statement 8 is difficult to u

Http://www.cnblogs.com/dubing/archive/2011/12/09/2278090.htmlRecently the company has come to a very tiger DBA 10几 years of experience here is called Cai Teacher. Let us share the precious wealth that Cai Lao has brought to us, with the consent of our Chua. Welcome other DBAs to shoot bricks Directory1. What is the execution plan? The execution plan is dependent on what information.2, unified SQL statement to reduce parsing overhead3. Reduce the

Directory1. What is the execution plan? The execution plan is dependent on what information.2, unified SQL statement to reduce parsing overhead3. Reduce the nesting of SQL statements4. Staging intermediate results using temporary table5. The OLTP system SQL statement must take a binding variable6. The spy problem of the bound variable of the tilted field7. Begin

a.cpu_time Desc Reference: http://jenniferok.iteye.com/blog/700985Query the most resource-intensive query from V$sqlarea Select B.username username,a.disk_reads reads,A.executions Exec,a.disk_reads/decode (a.executions,0,1,a.executions) Rds_exec_ratio,A.sql_text StatementFrom V$sqlarea A,dba_users bwhere a.parsing_user_id=b.user_idand A.disk_reads > 100000ORDER BY a.disk_reads Desc; Replacing the Disk_reads column with the buffer_gets column gives

simple addition relationship between them. So what is the Query Process on the server? First, SQL Server checks whether the data required to complete the query is in the data buffer. It will soon find that the data is not in the data buffer, the pre-read mechanism is enabled to read the first nine of the 10 Data Pages

learn. The learning curve is very flat, and it takes almost no time to write sq queries. SQL follows the "learn once, anytime, anywhere" principle, so it's a bargain to spend time learning SQL! SQL is an excellent complement to programming languages, and in some cases, writing queries is even more of a priority than writing code! ...

concatenated with "=" 5.2 Multi-line subqueries Sql>select Ename,job,sal,deptno from EMP where job in (SELECT DISTINCT job from EMP where deptno=10); the same worker in the query table that works with department Number 10 The name, work, salary and department number of the worker. Because there are multiple rows of returned results, the subquery is concatenated with "in". In and exists: the subquery fol

Wed Aug 20 17:16:37 2008ORA-1652: unable to extend temp segment by 128 in tablespace DBA_TEMP To solve this problem, we need to first cause the problem of SQL, there are several possible methods: 1. Set events Alter system set events '1652 trace name errorstack level 1 '; This method has some limitations: 1) it cannot obtain the error message of 1652 that has occurred. It can only generate a trace file when a 1652 error occurs in the future; 2) with e

Wed Aug 20 17:16:37 2008 ORA-1652: unable to extend temp segment by 128 in tablespace DBA_TEMP To solve this problem, we first want to cause the problem of SQL, there are several possible methods: 1, set events Alter system set events ' 1652 trace name errorstack level 1 '; This approach has certain limitations: 1 It cannot get 1652 error messages that have occurred and can only generate a trace file when 1652 errors occur later; 2) with events,

required to complete the query is in the data buffer. It will soon find that the data is not in the data buffer, the pre-read mechanism is enabled to read the first nine of the 10 Data Pages required by the system to the data buffer zone. When SQL Server checks whether all the required data is already in the data buffer, it will

In the face of rapid business development, one of the important tasks of DBA is to discover the inefficient SQL statements in the database in time, some can immediately start to solve (such as lack of appropriate indexes), and some need to feedback to the developer to make changes as soon as possible. The MySQL database has several configuration options to help us capture inefficient SQL statements in a ti

query time is not accurate, there are the following two aspects:1.SQL server adjusts itself as the server resources change.Because we usually test the environment of the server and the actual server is not exactly the same. For example, we test repeatedly on a server that is heavily loaded. You will find that the time of execution is not the same, of course, the

. Searching rows for update The qualifying records are being told to find out to prepare for the update. It must be completed before the UPDATE will modify the relevant records. Sleeping Waiting for client to send new request System Lock Is waiting to get an external system lock. If you are not currently running multiple mysqld servers requesting the same table at the same time, you can suppr

Query a stored procedure sharing of SQL Server database deadlocks. SQL Server Stored Procedure When SQL Server is used as the database application system, it cannot avoid deadlocks. When deadlocks occur, maintenance personnel or developers only use sp_who to find deadlocks,

Tags: des style blog http io color ar os use [MySQL Optimizer]--How to find the reason for SQL efficiency underground Source: Chinaunix Blog Date: 2009.07.20 16:12 (Total reviews ) I want to comment After querying to inefficient SQL sta

For optimization, finding SQL that consumes more resources is critical, and here are a few of the previously used SQL.1. Query the most resource-intensive query from V$sqlarea.Select B.username Username,a.disk_reads reads,A.executions Exec,a.disk_reads/decode (a.executions,0,1,a.executions) Rds_exec_ratio,A.sql_text St

limitations of this method are: 1) It is difficult to know the SQL Execution time in the V $ SQL view, and it is difficult to confirm that the specific SQL statement causes errors. 2) the SQL statement that causes the problem is probably out of age. 3. Generate the awr and statspack reports when an error occurs.