how to use count

Let's take a look at the description of Count (*) and Count (col) in Bol:COUNT(*Returns the number of items in the group. IncludingNULLvalues and duplicates. COUNT( Allexpression) evaluates expression for each row in the group and returns the number

If your need is to count total rows, why use COUNT (*) and avoid specifying specific column names? The count () function evaluates to the number of times a value item is evaluated when the argument is a column name. That is, items that have no value

Use smart pointers to simplify memory management. In the case of a tree, if you use a normal pointer, you usually call delete in the destructor when you insert a new node, but with a smart pointer of type UNIQUE_PTR, you do not need to delete it in

There are indexes and error-proofing mechanisms on the surface of database tables, but a simple query can take a long time. Web applications may work well in the development environment, but they are equally bad in the product environment. If you're

II. Improving SQL statements 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 >

Memory mapping is one of the most interesting features of modern Unix systems. As for drivers, memory mappings can be implemented to provide direct access to device memory by user programs. A clear example of a mmap usage can be seen from a subset

Usually because of the database study is not deep, so in the interview asked some of the usual encountered problems incredibly can not be very sure to answer, really let oneself very angry!One of the things that impressed me this time is:when using

"MySQL must know" notes-partComments:Start to organize notes, record their own reading process of doubt and focus, later found that the record does not have much meaning, it is better to go directly to the original book, so give up.So the note is

ObjectiveTalking 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 appropriate data type for the field .... Do you really understand

--MAX,MIN,SUM,AVG,COUNT,NVL (,)--Aggregate function--Max: Max value--Max can take the maximum value of the number, or the maximum value of the string (alphabetical order), according to the actual meaning of the scene, it is best to appear in a SQL

This paper introduces the tuning method of MySQL, including slow log query analysis and explain query analysis SQL execution plan1. mysql optimization1, slow log query analysisYou first need to make some settings for the slow log, as follows:SHOW

I wrote an article about this before.Why does "original" use data indexing to improve efficiency? (This article provides an overview of MySQL) (update)This time, the main focus is on two common storage engines.We generally perform MySQL database

Reprint Please specify source: http://blog.csdn.net/guolin_blog/article/details/40614197In the previous article, we have learned all the usage of the Litepal query operation, and it is clear that Litepal has provided us with a very powerful query

Public Cursor query (string table, string[] columns, string selection, string[] Selectionargs, String groupBy, String Havi Ng, string by-clause, string limit)Query the given table, returning a Cursor over the result set.ParametersString table--------

ASP. NET and MSSQL high-performance paging, asp. netmssql high-performance The first is the stored procedure. Only the data that I need is retrieved. If the number of pages exceeds the total number of data records, the records on the last page are

1. Test Environment OS: LinuxDB: mysql-5.5.18Table: innodb Storage Engine The table is defined as follows: 2. test scenario and analysis [number of group_message records in the statistical table] (1) select count (*) Method (2) select count

The following articles mainly describe five useful methods for MySQL query optimization. Anyone familiar with SQL statements knows that if you want to operate a task, SQL statements can be written in many ways, but the query performance varies with

  If you have a good memory, remember that the most I mentioned in the linux Device Driver instance post is the character device driver, so today's block I/O layer is a device that corresponds to the character device driver. The most fundamental

The data that Sharedpreferences saves is primarily data that resembles the configuration information format, so the data he saves is primarily a simple k-v pair.The Sharedpreferences interface is primarily responsible for reading the application's

In PHP, the values stored in the array become array elements, the array has a one-dimensional array and a multidimensional array, and the index of each array element is also called the keyword, and access to the array elements can be indexed.