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Http://blog.chinaunix.net/uid-25120309-id-3801250.htmlI. AWK descriptionAwk is a programming language that is used to process text and data under Linux/unix. Data can come from standard input, one or more files, or the output of other commands. It

Full use of--------------------join------------ Outer Joins. An outer join can be a left outer join, a right outer join, or a full outer join. When you specify an outer join in the FROM clause, you can specify one of the following groups of keywords:

First, the optimizer basic knowledgeBefore Oracle executes a SQL, it first analyzes the execution plan of the statement and then executes the execution plan. The execution plan for parsing statements is performed by the optimizer (Optimizer). In

HBase provides a set of filters for filtering data that allows you to filter data on multiple dimensions (rows, columns, and data versions) of data in HBase, i.e. the data that the filter can eventually filter can be refined to a specific storage

The join condition can be specified in the From or WHERE clause, and it is recommended that the join condition be specified in the FROM clause. The WHERE and having clauses can also contain search conditions to further filter the rows selected by

Original address: https://www.cnblogs.com/qiuqiuqiu/p/6442791.html1= <> 之类的比较运算符）。包括相等联接和自然联接。 内联接使用比较运算符根据每个表共有的列的值匹配两个表中的行。例如，检索 students和courses表中学生标识号相同的所有行。 2、外联接。外联接可以是左向外联接、右向外联接或完整外部联接。 在 FROM子句中指定外联接时，可以由下列几组关键字中的一组指定：

Explain shows how MySQL uses indexes to process SELECT statements and join tables. Can help select better indexes and write more optimized query statements. Using the method, add explain to the SELECT statement.I. Explanation of the Explain column1,

Second, advanced keywords--Introduction to use8,top use (the TOP clause returns the number of records)Select Top Number|percentcolumn_name (s) fromtable_name orSELECT TOP 2 * fromPersons Example:Select Top 5 * fromYk_typk--Check only the first 5

Today, when looking at a data sheet of a legacy system, we find that the view that we look for is a full-out join, which results in some restrictive processing of the data records, and can also be achieved by setting the view tables as right outer

1. Referencing 2 tables (effects with inner join)1 SELECT from WHERE =2.INNER Join Join (returns a row if there is at least one match in the table)1 SELECT from INNER JOIN on = ORDER by Persons.lastname3.LEFT join: Even if there is no match in

INNER JOIN: Returns a row if there is at least one match in the table Left JOIN: Returns all rows from the table, even if there is no match in the right table Right JOIN: Returns all rows from the correct table even if there is no match in the

1. Key candidate key for primary key Super keyPrimary key:A combination of data columns or properties in a database table that uniquely and fully identifies the stored data object. A data column can have only one primary key , and the value of the

1. cd commandThis is a very basic, is also a common need to use the command, it is used to switch the current directory, its parameters are to switch to the path of the directory, can be an absolute path, can also be a relative path. Such as:

1 in the table, duplicate values may be included. That's not a problem, but sometimes you might want to just list different values (distinct).Keyword DISTINCT is used to return only different values.Grammar:SELECT DISTINCT column name from table

In fact, for fresh graduates interview, if your actual combat experience is insufficient, the interviewer will generally ask some basic questions. (This blog content answer is summed up some of the original author's knowledge,)1 . Differences

Null in mongodb finds all rows whose name is NULL in XXX, including rows that do not contain the name key value: [javascript] db. XXX. find ({name: null}); find all rows whose name is NULL in the XXX set, excluding the rows that do not contain the

SQL Outer Join left on right on SQL Outer Join left on right on I. join) Through join, you can retrieve data from two or more tables based on the logical relationship between tables. Join indicates how to use data in one table to select rows in

[Reading Notes] Linux commands, editors, and shell programming-general Linux technology-Linux programming and kernel information. Read the following for details. Author: Hong haihui Mark G. Sobell of Linux Command, editor and shell programming

1. 'essential' of a trigger ': A trigger is a special stored procedure. It cannot be explicitly called. Instead, it inserts records, changes records, or deletes records into a table. When an event occurs, automatically activated. 2. The 'function ':

Function Description: Search for matching strings in the file.   Syntax:Grep [-Abcefghhillnqrsvvwxy] [-A ] [-B ] [-C ] [-D ] [-E ] [-F ] [-- help] [template style] [file or folder...] Note:GrepCommand is used to search for files whose content