sql count distinct values

Note 1 For mysql count distinct null: An example is used to explain the problem. Now another database table is hello. The table content is as follows: www.2cto.com id name 1 Null2 Null3 Null4 Null5 Null hello table has two fields: id and name, name is null. The first SQL statement www.2cto.com: SELECT COUNT (id) FROM h

of mysql is of much unexpected use.1. It can be used when count does not repeat recordsFor example, select count (DISTINCT id) FROM tablename;Calculate the number of records with different IDs in the talbebname table. 2. You can useFor example, select distinct id FROM tablename;Returns the specific

This article mainly compares the usage of distinct and count (*) in MySQL, and analyzes the usage and efficiency of the two in detail, interested friends can refer to some usage of MySQL DISTINCT keywords first: 1. it can be used when count records are not repeated. for example, select

MYSQL row-to-column conversion and basic aggregate function count, used in combination with group by and distinct, countdistinct The count function is often used in statistical queries. Here, the basic MYSQL row-to-column conversion and basic aggregate function count are used in combination with group by and

, and can not return to other fields, this problem let me worry for a long time, with the distinct can not be resolved, I only use the double loop query to solve, and so for a very large amount of data station, it will undoubtedly directly affect the efficiency. So I spent a lot of time studying this. MySQL's distinct keyword has a lot to do with your unexpected usefulness.1. Can be used when

IDs in the talbebname table. 2. you can use For example, select distinct id FROM tablename; Returns the specific values of different IDs in the talbebname table. 3. in case 2 above, there will be ambiguity when you need to return results with more than two columns in the mysql table For example, select distinct id, type FROM tablename; In fact, the r

1. Count: Copy Code code as follows: --In an empty collection, Count returns a number of 0. > Db.test.count () 0 --Test the return value of count after inserting a document. > Db.test.insert ({"Test": 1}) > Db.test.count () 1 > Db.test.insert ({"Test": 2}) > Db.test.count () 2 --count

Suddenly, it is found that the different processing of count (*) In the mysql tutorial will lead to different results. For exampleSelect count (*) from tablenameEven for mysql with tens of millions of data, results can be returned very quickly.ForSelect count (*) from tablename where .....Mysql query time began to rise Check the manual carefully and find that whe

1. Count:--In an empty collection, Count returns a quantity of 0. > Db.test.count ()0--Test the return value of count after inserting a document.> Db.test.insert ({"Test": 1})> Db.test.count ()1> Db.test.insert ({"Test": 2})> Db.test.count ()2--count and find also accept the conditions. As you can see from the results,

I want to count the number of fields a in a table. Because field a has repeated records, I want to exclude repeated records. The SQL statement is: Select count (distinct) Link: http://www.w3school.com.cn/ SQL/SQL _func_count_dist

,, Example: The difference between the two approaches is that the first method is to encapsulate the second method, the first one only returns the value collection of the field values after the redo statistic, but the second method returns both the field value collection and the detail information when the statistic is returned. (3) Group Function: Used to provide richer statistical requirements than count