t3 concentrator

the gid and cgid are different, the Group ID has changed and the Rank sorting is reset.In this way, Rank is auto-incrementing according to each group, and the first N records are enough. SELECT t3.tid, t3.stat, t3.createtime FROM (SELECT @ gid: = @ cgid, @ cgid: = t1.stat, if (@ gid = @ cgid, @ rank: = @ rank + 1, @ rank: = 1) AS rank, t1 .* FROM (SELECT * FROM

1. Introduction of Thread poolBenefits of Introduction:1) Improve performance. Create and consume objects cost-consuming CPU resources2) Prevent excessive memory consumption. Controls the number of active threads, preventing too many concurrent threads.Conditions of Use:Assume that the time to complete a task on a single server is TT1 when the thread was createdT2 time to execute a task in a thread, including the time required to synchronize between threadsT3 the time the thread was destroyedObv

only troublesome to use ON instead of writing. 2. NATURAL [LEFT] JOIN: This sentence serves as inner join, or contains Left JOIN (left join) of all fields in the joined table in the USING clause ). 3. STRAIGHT_JOIN: by default, MySQL first reads the left table during table join. When this parameter is used, MySQL reads the right table first, this is a built-in Optimization Parameter of MySQL. You should use it in certain circumstances. For example, if you have confirmed that the number of recor

1. Introduction of Thread poolBenefits of Introduction:1) Improve performance. Create and consume objects cost-consuming CPU resources2) Prevent excessive memory consumption. Controls the number of active threads, preventing too many concurrent threads.Conditions of Use:Assume that the time to complete a task on a single server is TT1 when the thread was createdT2 time to execute a task in a thread, including the time required to synchronize between threadsT3 the time the thread was destroyedObv

following course scores of 3rd to 6th students: Enterprise Management (001), Marx (002), UML (003), Database (004)[Student ID], [Student name], enterprise management, Marx, UML, database, average scoreSelect distinct top 3SC. S # As student ID,Student. Sname AS Student name,T1.score AS enterprise management,T2.score AS Marx,T3.score as uml,T4.score AS database,ISNULL (T1.score, 0) + ISNULL (T2.score, 0) + ISNULL (

= obj.parentelement; if (obj.tagname = = ' A ' | | | obj.tagname = = ' body ') break; } if (obj.tagname = = ' A ' amp;amp; obj.href!= ') { act = 1; clrorg = Obj.style.color; Timerid = SetInterval ("ChangeColor ()", 100); } } } function Stoprainbowanchor () { if (Browser amp;amp; Act!= 0) { if (obj.tagname = = ' A ') { obj.style.color = clrorg; clearinterval (Timerid); act = 0; } } } function ChangeColor () { Obj.style.color = Makecolor (); } function Makecolor () { if (El

statement, as in the following example: , which acts as the following statement A left JOIN B on A.c1=b.c1 and A.C2=B.C2 and a.c3=b.c3 Just using on instead of writing is more cumbersome. 2. NATURAL [Left] Join: This sentence acts as a inner join, or as a left-hand join with all fields in a table containing joins in a using clause. 3. Straight_join: By default, MySQL reads the left table first when it joins the table. When you use this parameter MySQL will read the

OK, 4 rows affected, 2 warnings (0.00 sec) records:4 duplicates:0 warnings:2mysql> SELECT * FROM t2;+------+| 0 | -1 saved as a 0| 0 | #无符号, the minimum value is 0| 255 | #无符号, the maximum value is 255| 255 | #256存成了255 +------+4 rows in Set (0.00 sec) ============ signed and unsigned int============= #int默认为有符号mysql > CREATE table t3 (x int) ;mysql> insert into T3 values, ( -2147483649), ( -214748364

the inner layer as the query condition of the outer querySelect field List from table name where condition (select field List from table name where condition);Select name from Db3.user ORDER BY uid desc LIMIT 1;Select AVG (UID) from Db3.user;Select Name,uid from Db3.user where uidSelect name from Db4.t1;Select name from Db3.user where name in (select name from DB4.T1);Format 3 Connection QueryLeft JOIN query: The table on the left is the main display of query resultsSelect field List from table

Dept_name is Depta, sort by ID in ascending order, otherwise by region_id ascending orderSELECT id,dept_name,region_idFrom T2ORDER by DECODE (Dept_name,' Depta ', ID,REGION_ID);3. Using decode to implement fixed row to column-Simple Employee work statisticsDROP TABLE T3;CREATE TABLE T3(Student_no Number (10),Student_name VARCHAR2 (10),Course_type VARCHAR2 (10),Course_score Number (10));INSERT into

Oracle in this case with the following experiment.First set up the relevant test table:ALTER DATABASE Add supplemental log data (primary Key,unique index) columns;CREATE TABLE test (t1 int, t2 int, t3 int, T4 int);ALTER TABLE TEST ADD constraint Pk_t1 primary key (T1); – Add primary KeyThen use a loop to insert a certain amount of dataUpdate test set t2=10; Commit -Update dataUsing the LOGMNR tool to analyze previous operations, you can see that the

multiple tables may be wrong in the way of sub-join queries. For example, you need to make a multi-table connection, so you enter the following junction query:SELECT t1.id,t2.id,t3.idFrom T1,t2Left JOIN T3 on (t3.id=t1.id)WHERE t1.id=t2.id;But MySQL does not do this, the actual execution of the background is the following statement:SELECT t1.id,t2.id,

when C # = ' 004 ' then score ELSE 0 END)/sum (Case C # when ' 004 ' then1 ELSE 0 END) as database average score, * SUM (case when C # = ' 004 ' and score >= 1 ELSE 0 END)/sum (caseWhen C # = ' 004 ' then 1 ELSE 0 END) as database percent passedFrom SC21, query different teachers teach different courses average from high to low displaySELECT Max (z.t#) as Teacher Id,max (Z.tname) as faculty name, c.c# as course ID,max (C.cname) As course name, AVG (score) as average scoreFrom SC as t,course as

TABLE T1;Mode two, direct use of the command Hadoop FS-PUT/USR/LOCAL/ID/HIVE/T1The above ID file is on Hadoop locally.Create a table of multiple fields T2CREATE TABLE T2 (ID int,name string) from the FORMAT delimited fields TERMINATED by ' \ t ';Then create a Stu file in Hadoop local, such as the/usr/local/directory, which contains two fields, separated by a tabThen add to the day T2 in Hadoop fs-put/usr/local/stu/hive/t2Querying in Hive, except for the SELECT * FROM statement, all the other st

Command 1:which Which command to find only the directory where the command is located For example: which LS Command 2:whereis Whereis command to find only the directory where the command is located + Help document location Whereis command Command location + Help document location Command 3:find Find path Lookup method parameter to locate any file There are a number of ways to find: 1. Find by file name Find how:-name For example: Find/t3-n

named in the main Q Uery as if they were "real" tables: WithSubquery_nameAs(The aggregation SQL statement)SELECT(query naming subquery_name); Retuning to We oversimplified example, let's replace the temporary tables with the SQL with "clause": ================================================================================= Below oneself small try a hand, of course, is not complex at all, very simple very simple example, hehe. Sql> CREATE TABLE t2 (id int); Table created. sql> CREATE TABLE

Tutorial_count is not NULL;+-----------------+----------------+| Tutorial_author | Tutorial_count |+-----------------+----------------+| Mahran | 20 || Gill | 20 |+-----------------+----------------+2 rows in Set (0.00 sec) subquery not in and NULL in the not EXISTS In some cases, a subquery of not in form returns an empty result set, but it is returned to normal after overwriting it to the not EXISTS form, as follows:Build table: The code is as follows Copy Code

the week%x Local corresponding date representation%x Local corresponding time representation%Z the name of the current time zone%%% per se 9.strptime (...)Strptime (string, format)-> Struct_timeConverts a time string to an array of times based on a specified format characterFor example:2009-03-20 11:45:39 The corresponding format string is:%y-%m-%d%h:%m:%sSat 28 22:24:24 2009 The corresponding format string is:%a%b%d%h:%m:%s%Y 10.time (...)Time ()-> floating point numberReturns the time stamp

Startofayear ... Startoftheyear ... Endofayear ... Endoftheyear ... The start and end times of each year, month, week, day {参数是指定的年、月、周、日} DateUtils.StartOfAYear DateUtils.StartOfAMonth DateUtils.StartOfAWeek DateUtils.StartOfADay {参数是　TDateTime} DateUtils.StartOfTheYear DateUtils.StartOfTheMonth DateUtils.StartOfTheWeek DateUtils.StartOfTheDay {参数是指定的年、月、周、日} DateUtils.EndOfAYear DateUtils.EndOfAMonth DateUtils.EndOfAWeek　 DateUtils.EndOfADay {参数是　TDateTime} DateUtils.EndOfTheYear DateUtils

this case with the following experiment. First set up the relevant test table: ALTER DATABASE Add supplemental log data (primary Key,unique index) columns;CREATE TABLE test (t1 int, t2 int, t3 int, T4 int);ALTER TABLE TEST ADD constraint Pk_t1 primary key (T1); --Add primary keyThen use a loop to insert a certain amount of dataUpdate test set t2=10; Commit --Update data Using the LOGMNR tool to analyze previous operations