how to use count

drop database if exists SS;create database SS;use SS;create table Student ( Sno char(9) primary key, Sname char(20) unique, Ssex char(2), Sage smallint, Sdept char(20));create table Course( Cno char(4) primary key, Cname char(40), Cpno char(4)

On the surface of the database table, there is an index and error prevention mechanism. However, a simple query takes a long time. Web applications may run well in the development environment, but are equally bad in the product environment. If you

This article mainly to share with you how PHP implementation of multi-file upload, I hope to help everyone. PHP File Upload Process1. Click the Submit button and the browser user submits the form data containing the uploaded file to the PHP handler2

Introduction: from 2.4 to 2.6, the Linux kernel has greatly changed in terms of the module mechanism, device model, and some core APIs that can be loaded, device driver developers are faced with porting drivers from 2.4 to 2.6 kernels, or enabling

Mysql database SQL optimization principles (experience summary) bitsCN.com I. prerequisites The principle here is only for MySQL databases. some of the other databases are similar, and some are still different. I have summarized the general rules

Is it better to use UNIQUE to insert data, or is it more efficient to use count (*) to query data before insertion ?? You cannot insert multiple & nbsp; title & nbsp; + & nbsp; URLs & nbsp; in a table with the same data. Method A: & nbsp; use & nbsp;

Smarty template to determine the method of empty array Here are two main ways of doing this: 1. Use count to get the number of subscripts in the array In the following example, if $array is empty, no data is output 123 {if $array |

IE, as long as the mouse floating on the status of the person's name, the state is connected to Lync, IE will appear to stop working.Here's how to fix it.Until The other day if I figured this off, every time I hovered my mouse pointer over a

13.5 manage pointer membersWhen copying a pointer, only the address in the pointer is copied, instead of the object pointed to by the pointer.Most C ++ classes use one of the following three methods to manage pointer members:(1) pointer members

Replication control-manage pointer members Introduction: For classes that contain pointers, pay special attention to the replication control. The reason is that the address in the pointer is copied only when the pointer is copied, and the object

Use of C ++ 11 smart pointer unique_ptr -- Take sorting binary tree as an ExampleYou can use smart pointers to simplify memory management. Take the tree as an example. If a common pointer is used, it is usually used to insert a new node and call

Create a data table IP address, access time, and access count. If you insert a record every time you access the database, the AccessCount is not required. You can use count for queries. This puts a lot of pressure on the database when the traffic is

  Reprint Please indicate the source, the original address: http://blog.csdn.net/hackbuteer1/article/details/7561235Smart pointer (smart pointer) is a class that stores pointers to dynamically allocated (heap) objects and is used for lifetime

C + + Enumeration typeIf a variable has only a few possible values, it can be defined as an enumeration (enumeration) type. The so-called "enumeration" refers to the value of the variable enumerated, the value of the variable can only be listed in

SELECT * FROM table1 where name= ' Zhangsan ' and TID > 10000and implementation:SELECT * FROM table1 where TID > 10000 and name= ' Zhangsan 'Some people do not know the execution efficiency of the above two statements is the same, because if it is

Many people do not know how SQL statements are executed in SQL Server, fearing that their written SQL statements will be misunderstood by SQL Server. Like what: SELECT * FROM table1 where name= ' Zhangsan ' and TID > 10000 and implementation:

This article is "high-performance MySQL" Reading notes Slow Query Basics: Optimizing data accessThe most basic reason for poor query performance is that there is too much data to access. For inefficient queries, we found that the following two

1. Oracle Naming conventions: Similar to Java naming Conventions 1. Strictly case-sensitive 2. All SQL statements must be '; ' End 3. All SQL is separated by a space: Sqlplus space/space as Space sysdba carriage return 4. Either the

This article transferred from: HTTPS://WWW.TUICOOL.COM/WX/2EMBFMQObjectiveTalking about MySQL query optimization, I believe you have a collection of artifice: You cannot use SELECT *, do not use null fields, reasonably create an index, select the

Indexes and error-proof mechanisms exist on the surface of a database table, but a simple query can take a long time. Web applications may run well in the development environment, but they behave equally poorly in a production environment. If you