Mysql Select And Count

First, the importance of the index The index is used to quickly find a column in a particular value of the line. Instead of using an index, MySQL must start with the first record and then read the entire table until it finds the relevant row. The larger the table, the more time it takes. If the table in the query column index, MySQL can quickly reach a location to search the middle of the data file, there is no need to see all the data. Note that if you need to access most of the rows, sequential reads are much faster since we avoid disk searches. If you use Xinhua Dictionary to find "Zhang" the Chinese characters, do not use the directory, then ...

SQL used to manipulate the database is generally divided into two kinds, one is the query, which is what we call the SELECT statement, the other is to update the statement, also known as data manipulation statements. The implication is to modify the data. There are 3 statements in the standard SQL, which are INSERT, UPDATE, and DELETE. There is another REPLACE statement in MySQL, so this article uses MySQL as a background to discuss how to make updates in SQL. First, INSERT and REPLACE I ...

Operation database is the basis of dynamic Web programming, this article gives you a detailed description of how PHP operates MySQL database 1. Establish and close connection 1 mysql_connect () resource mysql_connect ([string hostname [:p ort][:/path/to/socket][,string username] [, String Passwor ...

Today found that the deder MYSQL time field, are used `senddata` int (10) unsigned NOT NULL DEFAULT '0'; Then find this article online, it seems that if the time field is involved in the operation, int better, a To retrieve the field without conversion operations, directly for time comparison! Second, the efficiency is higher as described below. In the final analysis: int instead of the data type, more efficient. Environment: Windows XP PHP Versio ...

This cost me a lot of time, the use of php, my head is mysql, it has been affected by a paging category, access is not supported limit page, like the same key, the problem solved immediately. Look at the comments should be very easy to understand, access to the paging sql from asp json google code up 001 <? Php 002 / ** 003 // paging class 004 * / 00 ...

The MySQL tutorial does not use the rand () function to randomly read the database tutorial records, as well as Google the relevant files, found that everyone almost exclusively using ORDER by RAND () to achieve this, but in fact there are very serious performance problems. If your database has only hundreds of, and the number of calls is not many times, you love what method to use what method. But if you have 100,000 or 1 million or more data, then each time you execute the SQL with ORDER by rand () ...

Mysql Guide sql distinct delete duplicate records How to use distinct keyword in mysql Filter the result list in a select statement Duplicate value If you want to delete duplicate data query data, you can use the distinct keyword to filter duplicate values, the following distinct usage. select distinct column_name from table_name; Let's take a look at a simple example ...

Method 1 delete yourtable where [id] not in (select max ([id]) from yourtable group by (name + value) b on a.id = b.id where b.id ...

Simple mysql tutorial grouping statistics statement instance code / id name code_id code_name is_true score 1 Zhang San 1 handsome is 5 2 Zhang San 2 lavish is 4 3 Zhang San 1 handsome is 4 4 Zhang San 2 lavish no 3 5 Li Si 1 handsome Is 5 6 Li Si 2 lavish is 3 to see an instance mysql grouped by name statistics: [handsome] how many ...

1> explain SQL, similar to Oracle explain statement For example: explain select * from nad_aditem; 2> select benchmark (count, sql); Calculate the sql statement to perform count times For example: mysql> select benchmark (1000; Select * FROM AD;); + ---------...