select count distinct db2

Wang ', 'First ban', 'female, 0 ); insert into 'tb _ student 'VALUES ('4', 'xiaojun', 'zhang ', 'class 2', 'mal', 0 ); insert into 'tb _ student 'VALUES ('5', 'xiaofang', 'zhang zhang', 'second class', 'female, 0 ); insert into 'tb _ student 'VALUES ('6', 'xiaoqiang', 'old Wang ', 'second class', 'mal', 0 ); insert into 'tb _ student 'VALUES ('7', 'dana', 'old Wang ', 'second class', 'female, 0 ); insert into 'tb _ student 'VALUES ('8', null, null ); * // **************************/explain

Research on the usage and efficiency of count (*) and DISTINCT in Mysql The usage and efficiency of count (*) and DISTINCT in Mysql When processing a large data database Suddenly, it is found that different mysql processing of count (*) results in different results. For

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

Hive processes count distinct to produce data skewProblem description The problematic data is skewed to the category, but it cannot be joined on the Map side, and special keys are excluded for processing. set hive.groupby.skewindata=true;insert overwrite table ad_overall_day partition(part_time='99', part_date='2015-11-99') select account_id, nvl(client_id,-1), n

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

Label:When working on a large data volume databaseIt suddenly turns out that MySQL's different handling of count (*) can result in different results such as executionSELECT Count (*) from TableNameEven for TENS data, MySQL can return results very quickly.And forSELECT Count (*) from TableName WHERE .....MySQL query time starts to climb Read the manual carefully a

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

The double count (distinct) here refers to a statement similar to the following Select Day,count (Distinct session_id), COUNT (distinct user_id) from log a group by; If you want to ex

I suddenly find that the MySQL tutorial will produce different results for the different processing of count (*) such as executionSelect COUNT (*) from TableNameMySQL can return results very quickly even for tens other data.And forSelect COUNT (*) from TableName where .....MySQL query time starts to climb Check out the tired manual and find that when there is n

Tags: BSP rom collect tin hash pos data Select MicOriginal address: Hybriddb Performance Optimization · Several implementation methods of Count distinct HYBRIDDB is an MPP analytic database developed by Ali based on Greenplum, and Greenplum itself is based on PostgreSQL. Thus, Hybriddb's optimization ideas and means will inevitably be affected and limited by Pos