tricky sql queries

Tags: mysql optimized sql slow logRecently, a development team complained that our platform contains MySQL cluster not, always reported the MySQL "Heartbeat Error", analyzed the logs they collected, did not find the network between the MySQL cluster node problem, but found a very interesting phenomenon , in the MySQL slow log found a large number of slow queries, basically is more than 1 seconds, it is obvi

, if not added distinct the result is the number of table rows--5.returns the column total value (SUM):Note: Sum as long as all with distinct two count specification, none *. Calculate the sum of students ' ages:SELECT SUM (student_age) from T_student;Return column average (AVG):Calculate the average age of students:SELECT AVG (student_age) from T_student;Returns the maximum/minimum value (max/min):Ask for the oldest student information (minimum value same):SELECT MAX (student_age) from T_studen

Several friends have such questions: select * from tab1, tab2 where tab1.id = tab2.id and tab1.col1 = 123 and tab2.col1 = 'abc' According to the execution order you described, tab1 and tab2 are first given the Cartesian product, and then filtered by tab1.col1 = 123 and tab2.col1 = 'abc. In this case, the efficiency is not very low. Is the database so stupid?I think many people will have this question, including the question I raised when I first studied. In this article, I will

;string sql = "SELECT * from user where username=?" and passwd=? "; PS = conn. PreparedStatement (SQL);p s.setstring (1, "admin");p s.setstring (2, "123456"); ResultSet = Ps.executequery (); Specific implementation Code reference java PreparedStatement (Java Platform SE 7) nbsp, other languages can be implemented against his principles. Parameter queries are the

SQL language is a simple but powerful language that allows you to quickly get started and write more complex query statements. But for most developers, using SQL to query a database doesn't have an abstract process and a reasonable step, this is likely to lead to a "card" when writing specific SQL query statements to solve specific problems, and this series focus

Cause: "SQL Server blocked access to component ' Ad Hoc distributed Queries ' STATEMENT ' Openrowset/opendatasource" when replicating data from a remote server to local Because this component has been shut down as part of this server's security configuration. System administrators can enable ' Ad Hoc distributed Queries ' by using sp_configure. For more informati

When the analyzer looks for a path, it does not count the number of rows and ranges of values contained in the index every time, instead, the information is created and updated based on certain conditions and saved to the database. This is the so-called statistical information. How to view statisticsIt is very simple to view SQL Server statistics. Run the following command:DBCC SHOW_STATISTICS ('table name', 'index name ') The result 1 is displayed.

SQL Server blocks access to the statement 'openrowset/OpenDataSource 'of the 'ad hoc distributed queries' component because this component has been disabled as part of the server's security configuration. The system administrator can enable 'ad hoc distributed queries 'by using sp_configure '. For more information about enabling 'ad hoc distributed querys', see "

Recursive queries in SQL statements SqlServer2005 and Oracle two versions Before using Oracle, think of its recursive query is very useful, the study of SQL Server, found that it also supports recursive query in SQLTo illustrate:The SqlServer2005 version of SQL is as follows:For example, a table with ID an

service to perform full-text indexing and querying, consider: Configure the virtual memory size to be at least 3 times times the physical memory installed on the computer. Configure the SQL Server max server memory server configuration option to 1.5 times times the physical memory (half of the virtual memory size setting). 7. Increase the number of server CPUs, but it is important to understand that parallel processing of serial processing requires r

for queries. SQL queries are optimized based on the data in the table. When there is a large number of duplicate data in the index column, SQL queries may not use indexes, for example, if a table contains sex fields, male and female are almost half of each other, indexing s

SQL Server blocks access to component \ 'ad Hoc Distributed Queries \' An error occurred while querying the Excel file in SQL Server:SELECT * from openrowset ('Microsoft. JET. OLEDB.4.0 ', 'excel 8.0; IMEX = 1; HDR = YES; DATABASE = D: \ a.xls', [sheet1 $])Result prompt:SQL Server blocks access to the STATEMENT 'openrowset/OpenDatasource 'of the 'ad Hoc Distribut

consume system resources, should be changed to this:Create table # t (...) 13. in many cases, replacing in with exists is a good choice:Select num from a where num in (select num from B)Replace the following statement:Select num from a where exists (select 1 from B where num = a. num) 14. not all indexes are valid for queries. SQL queries are optimized based on

Label:I. Preface:Typically, queries on tables in a project are associated with multiple tables, and multi-table queries involve intra-and outer-join and self-connected queries of SQL. This article will briefly describe the use of these SQL connections, we hope to help.Two. D

Transferred from: http://blog.sqlauthority.com/2010/05/14/sql-server-find-most-expensive-queries-using-dmv/The title of this post was what I can express here for this Quick blog post. I was asked in recent Query tuning consultation project, if I can share my script which I use to figure out which is the M OST expensive queries is running on

Label:Oracle queries recently executed SQL statements select Sql_text,last_load_time from V$sql order by last_load_time Desc;SELECT Sql_text, last_load_time from V$sql WHERE last_load_time are not NULL and sql_text like ' select% ' ORDER by Las T_load_time DESC; SELECT Sql_text, last_load_time from V$

The SQL Server database administrator often struggles to clean up the table and write many queries to find the hard disk space used by the table. This article describes how to query the space usage of tables and help the database administrator identify the most used space to archive old data and clear unnecessary data tables. 1. Log on to S The SQL Server databas

join local table B on A.column1=b.column1--2, OpenQuery usage need to create a connection--First create a connection to create a linked serverexec sp_addlinkedserver ' itsv ', ' ', ' SQLOLEDB ', ' Remote server name or IP address ' 　　　　 --QuerySELECT * FROM OPENQUERY (ITSV, ' SELECT * from database. dbo. Table name ')--Import local tables to remote tablesInsert OpenQuery (ITSV, ' SELECT * from database. dbo. Table name ')SELECT * FROM local surface--Update the local surfaceUpdate b set B. colu

The SQL statement generated when using the generic data type in the Linq To EF clause queries the entire table.1. Symptoms public class LinqHepler [HttpGet] [Route ("LinqWhere")] public IActionResult LinqWhere () {string uname = ""; _ context. userInfo. where (c => c. userName = uname ). toList (); // LinqHepler with conditions Production SQL 2017-10-11T02: 59

Reference: http://blog.chinaunix.net/u/3866/showart_396335.html----------------------- V $ sqlarea, V $ SQL ----------------------- Select B. Username username, A. disk_reads reads, A.exe cutions exec, A. disk_reads/decode(a.executions,, a.exe cutions) rds_exec_ratio, A. SQL _text statement From v $ sqlarea A, dba_users B Where a. parsing_user_id = B. user_id And a. disk_reads> 100000 Order by A. disk_reads