select count distinct

--Check the frequency of occurrenceSelect COUNT (*) as Cnt,cityFrom Sakila.city_demo GROUP by city ORDER by CNT DESC LIMIT 10;1.select count (Distinct city)/count (*) from Sakila.city_demo;* Selection of complete columns2. Select COUNT (distinct

One: type of indexTwo: Advantages of the IndexThree: High performance indexing strategyFour: Index case1.1 Type IntroductionThere are many types of indexes that can provide better performance for different scenarios. In MySQL, indexes are

Logic is simpler, directly on the code Timed delivery uses the win server's timed task to execute the script regularly #coding: Utf-8 __future__ Import Division import PYMSSQL,SYS,DATETIME,XLWT Import smtplib from Email.mime.text Impor T mimetext

When you index a table, some of the queries will use the order by, the group by, and so on, and in a lower version (with a prior version of ICP), you may experience a situation where the query uses the index, so the order by cannot use the index,

Copy Code code as follows: --sql Basic Operations --Create a database Create DATABASE Studets --Create a table CREATE TABLE Student (Sno char (5), sname char (), Ssex char (2), Sage smallint, sdept char (15)) CREATE TABLE Course

SQL SELECT DISTINCT Statement Grammar: SELECT DISTINCT column name from table name using DISTINCT keyword If you want to select all the values from the company column, we need to use the SELECT statement: SELECT Company from Orders To select

My WordPress database Wp_postmeta table has more than 10,000 lines, if the use of two cycles to find the judge, although we are not tired, but the program runs very tired AH. If I want to query with a single statement to get all the data that

This article describes Oracle's query optimizer, a key component of the database, which gives Oracle users excellent performance. Oracle's query optimization technology is unmatched in functionality, and this article discusses all the important

1. Use an SQL statement to find out the names of students who have more than 80 points per courseName Kecheng FenshuZhang San language 81Zhang San Mathematics 75John Doe Language 76John Doe Mathematics 90Harry Language 81Harry Mathematics 100Harry

Unique indexEmphasis is unique, that is, the index value must be unique.Create unique index [index name] on table name (column name);ALTER TABLE name add unique index [index name] (column name);To delete an index:Drop index index name on table

1. To create a normal User: Grammar:CREATE USER ' user ' @   ' Host ' [identified 'password']; User: username,Host: hostname,password: password; Eg : Create a local user Teacher, Password is 123456 ,CREATE USER ' teacher

Objective A few days to take a time to "high-performance MySQL" looked at the index is very important, and then summed up to consolidate knowledge. This article uses the InnoDB storage engine as an index. Because this article does not say that the

Source: https://www.cnblogs.com/linjiqin/archive/2012/01/15/2323030.htmlFirst, what is the waterline (high water Mark)?All Oracle segments (segments, where, for ease of understanding, suggest segment as a synonym for a table) have a limit on the

Prefix index, which means that the first n characters are generally selected as the index for a field of type Varchar/text/blob. Indexing a long character column can make the index big and easy. Some of the characters at the beginning of the index,

First, what is the waterline (high water Mark)?All Oracle segments (segments, where, for ease of understanding, suggest segment as a synonym for a table) have a limit on the amount of data that can be accommodated in a paragraph, we call this cap

Drop Table if existsStudent;Create TableStudent (Sidvarchar(5)Primary Key, Snamevarchar(Ten), Sageint(3), Ssexvarchar(Ten));Drop Table if existsCourse;Create TableCourse (Cidvarchar(5)Primary Key, Cnamevarchar(Ten), Tidint(5));Drop Table if existsSC;

Today, when querying a table row count, it is found that count (1) and COUNT (*) execution efficiency are the same. There's a difference between this and Oracle. Then look at the execution plan in two ways:Mysql> Select COUNT (1) from customer;+-----

Advanced queries are most frequently used in databases and are also the most widely used. For more information about how to learn sqlserver, see. Advanced queries are most frequently used in databases and are also the most widely used. For more

Databases have many processing models for hierarchies. You can design models based on your needs. Of course, the simplest and easiest model to design is the so-called adjacent model. In this regard, other databases, such as Oracle, provide the ready-

1. Start and exit 1. Go to MySQL: Start MySQL Command Line Client (MySQL DOS Interface) and enter the password for installation. The prompt is: mysql>2. exit MySQL: quit or exitIi. Database Operations1. Create a databaseCommand: create database For