how to write distinct query in sql

Optimizing SQL queries: How to write high-performance SQL statements1, first to understand what is called the implementation plan?An execution plan is a query scheme that the database makes based on the statistics of SQL statements and related tables, which are generated aut

Label:Next: T-SQL Dynamic Query (3)--Static SQLObjective: There's a lot of talk about dynamic queries, and this article describes some of the ways to use dynamic SQL to solve dynamic queries.Why use Dynamic sql: in many projects, dynamic SQL is widely used and even misused,

more of data for each data table design, and write out the SQL code that inserts data for the Stuinfo "Student Basics" Table:Stuinfo data for the "Student Basics" tableSchool Number (Stuno) name (name) gender .....2017010213 Wang Xiao Belle plaine 1999-02-12 ...Code:Task five, data query (5 points for each sub-question, total 30 points)1) Check the "Basic inform

First, simple query SQL (structured query Language) Structured Query language is a database query and programming language for accessing data and querying, updating, and managing relational database systems. ANSI (American National Standards Institute) claims that

, ThinkPHP enables dynamic queries. 1. getBy dynamic query // Find the data for email = xiaoin@163.com $ User = M ('user '); Var_dump ($ user-> getByemail ('xiaoxin @ 163.com ')); 2. getFieldBy dynamic query // Get the corresponding id value through user $ User = M ('user '); Var_dump ($ user-> getFieldByUser ('lufei ', 'id ')); 8. SQL

I just transferred to php. please help me write code for the segment database query class. Hello, I first switched to php and wanted to create a database operation class with a pdo connection, but I don't know how to call php, please give us some advice. Class file coon. php content: Query ("set names UTF8"); $ dbh-> exec ("set names UTF8");} catch (Exception

Next: T-SQL Dynamic Query (1)--IntroductionObjective:In the development of features, we often encounter scenarios such as the following: The application has a query function that allows the user to select the desired condition in many query conditions. This is also the focus of this series.But sometimes you may find th

WHERE EMPLOYEE.EMP_ID IS NULL And SELECT JOB_DESC FROM JOBS WHERE (SELECT COUNT(*) FROM EMPLOYEE WHERE JOB_ID=JOBS.JOB_ID) It should be written: SELECT JOB_DESC FROM JOBS WHERE EXISTS (SELECT 1 FROM EMPLOYEE WHERE JOB_ID=JOBS.JOB_ID) Try to use Indexes After an index is created, not every query uses an index. When an index is used, the efficiency of the index varies greatly. As long as the index is not forcibly specified in the

field) >1) 3. It's very easy to do a delete update on duplicate records. For example, keep only one of the minimum IDsDelete data table where ID in (the Select Max (ID) from packet Group by repeating record field has count (heavy Complex record field) >1)Update Operation No, it's the same. 4.group by field having count and distinct difference DISTCT query shows that all field values are the same, one recor

Analyzer is considered to be two different SQL statements, must be parsed two times. Generate 2 execution plans. So as a programmer, you should ensure that the same query statements everywhere are consistent, one more space is not! 3. Do not write SQL statements too complex I often see a

. Select*from dualSelect*from dual In fact, the case is different, the Query Analyzer is considered to be two different SQL statements, must be parsed two times. Generate 2 execution plans. So as a programmer, you should ensure that the same query statements everywhere are consistent, one more space is not! 3. Do not

1. What is SQL injectionSQL injection, by inserting a SQL command into a Web form to submit or entering a query string for a domain name or page request, eventually achieves a malicious SQL command that deceives the server.2. How to prevent SQL injectionThere are two ways to

Write efficient SQL statements and SQL statements (1) simple integration with no associated database access: If you have several simple database query statements, you can integrate them into a single query (even if there is no relationship between them) (2) Delete duplicate

should ensure that the same query statements in any place are consistent, one more space is not! 3. Don't write SQL statements too complex I often see a SQL statement captured from a database that prints 2 sheets of A4 paper so long. In general, such complex statements are usually problematic. I took the 2-page long

Is there any other way to write a query like this? PHPcodepublicfunctionfind () {$ SQL quot; SELECT * FROMqsj_user_profileRIGHTJOINqsj_userONqsj_user.uidqsj_user_profile.uid quot; is there any other way to write a query like this? PHP code public function find()

), variable length, can hold photos, files, etc. int, integer field. Example: Userage int float, floating-point data is approximate. Example: Userresults float, the approximate numeric data type used to represent floating-point numeric data. datetime, Date type. Example: Createdatetime datetime Bit, Boolean type. Can be set to True is male, false is schoolgirl If the content is very much, indeterminate length (such as article), can be used: nvarchar (max) 6. Save the tabl

How to compile efficient SQL query statements and SQL statementsOverview How can I write SQL query statements with high performance? Two Methods: creating reasonable indexes and writing efficient

Is there any other way to write a query like this? PHPcodepublicfunctionfind () {$ SQL = quot; SELECT * FROMqsj_user_profileRIGHTJOINqsj_userONqsj_use is there any other way to write a query like this? PHP code public function find() { $

Label:In SQL Server, each query will find the shortest path to achieve its own goal. If the database accepts only one connection at a time, only one query is executed. Then the query is, of course, M.F.B. s to complete the work. For most databases, however, multiple queries need to be processed at the same time. Instea

Optimization of SQL massive data query and non-use like processing solutions, data query like 1. To optimize the query, try to avoid full table scanning. First, consider creating an index on the columns involved in where and order. 2. Try to avoid null value determination on the field in the where clause. Otherwise, th