select count distinct

Recent BA user feedback has two seemingly very similar statements return the number of results is not the same, more strange, doubt is not hive bug Query 1 Returns the number of results 6071 Select COUNT (Distinct reviewid) as dis_reviewcnt

I. Why should I enable this query? Databases are prone to bottlenecks. Now Nosql is so popular that it is estimated that they are all depressed by databases. The statements that affect the query speed in mysql are very slow. These slow statements

Summary of Mysql slow query operations and summary of mysql Mysql slow query explanation The slow query log of MySQL is a log record provided by MySQL. It is used to record the statements whose response time exceeds the threshold value in MySQL,

[Oracle @ PD009 ~] $ Grep-iapprox_countoracle.strsettingsforapprox_count_distinctoptimizationsqkaGBPushdown: estimatedmemorywithoutGPDgroupSize (%. 2f) * aclsum (% u) %. 2f; estimatedmemorywithGPDoptHllEntry (% u) * # ofappro [Oracle @ PD009 ~] $

I wrote the second script after learning Python. The logic is messy and will be optimized later! #! Usrbinenvpythonauthor: wenminCreatedon2013-4-23? ImportMySQLdb? ClassMySQLHelper: # configure the database information and connect def _ init _ (self,

1. Accelerate SQL Execution 1. Use sort or join in the select statement If you have sorting and join operations, you can first select data to a temporary table and then process the temporary table. Because temporary tables are created in the memory,

50 commonly used SQL statements Student (S #, Sname, Sage, Ssex) Student table Course (C #, Cname, T #) curriculum SC (S #, C #, score) exam table Teacher (T #, Tname) Instructor table questions: 1. query the student ID of all the students whose

The Bulk Collect feature is introduced in Oracle8i for the first time. This feature allows you to use batch queries in PL/SQL. Batch queries can significantly improve query efficiency in some cases. Now, we will perform some simple tests and

50 common SQL statementsStudent (S #, Sname, Sage, Ssex) Student tableCourse (C #, Cname, T #) CurriculumSC (S #, C #, score) Orders tableTeacher (T #, Tname) Instructor tableProblem:1. query the student ID of all students whose score is higher than

Explicitly converts a data type expression to another data type. CAST and CONVERT provide similar functions. Explicitly converts a data type expression to another data type. CAST and CONVERT provide similar functions. SyntaxUse CAST: CAST

In pinwu performance optimization, Chapter 4.4.3 describes the two features of composite indexes: prefix and optionality. What is prefix? This book describes how to exclude skipscanindex (there are not many pre-index columns, such as gender). If the

{Code...} sometimes needs to look up the uid in a table like the one above, and use the count method of medoo. how can we achieve the functionality of the above statement? Select count (distinct (uid) FROM nazou. items_want Sometimes you need to

Write a SQL query to get the nth highest salary from the Employee table.+----+--------+| Id | Salary |+----+--------+| 1 | | | 2 | | | 3 | |+----+--------+For example, given the above Employee table, the n-th highest salary where n = 2 is 2

Let's take a look at the following section.Code Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/--> Oracle uses the bottom-up sequence to parse the WHERE clause. Based on this principle, the

Each login generates a log record containing the logon time and user's UserID How do I write SQL to find out how many people have logged in today? Duplicate UserID not logged? "select count(*) from log_login where login_time > '{date('Y-m-d')}'

Table A: ID Name banji_id//banji_id is the ID of table B 1 Zhang Long 1 2 Zhang Long 2 3 Li 41 3 Li 45 5 Wang 53 6 Zhao Hu 2 7 Zhao Hu 1 8 Zhaohu 4 Table B: ID km SJ1 SJ2 1 Abacus Mental Arithmetic 2015 spring 2 Eloquence 2015 Spring 3 Composition

Student (s #, sname, sage, ssex) Student table   Course (C #, cname, T #) Curriculum   SC (s #, C #, score) Orders table   Teacher (T #, tname) Instructor table       Problem:   1. query the student IDs of all students whose "course scores are

Http://blog.csdn.net/xys_777/archive/2010/06/22/5685953.aspx Summary of Row-to-column conversion problems-1. Row-to-column conversion (various problems in the forum will be sorted out later) --- 1. Simplest row-to-column Conversion/* Question:

-- Sample DataCreate Table Tb (name varchar (10), score decimal (10, 2 ))Insert TB select 'A', 99Union all select 'bb', 56Union all select 'cc', 56Union all select 'dd', 77Union all select 'ee ', 78Union all select 'ff ', 76Union all select 'gg ', 78

/* Table 1. employee table ): eid name department job email password 10001 Li Ming SBB eg 10003 Li Yiping Luke ITM 11045 Li Jie SBB eg 10044 Hu Fei MTD etn 10009 Xu zhonggang SBB eg 10023 Li Yan SBB etn 20460 Lu mingsheng MTD etn 20078 Zhang Qing