• MySQL explain [forward]


    1、 Grammar explain < table_name > For example: explain select * from T3 where id=3952602; 2、 Explain output interpretation +—-+————-+——-+——-+——————-+———+———+——-+——+——-+| id | select_type | table | type  | possible_keys     | key     | key_len | ref   | rows | Extra |+—-+————-+——-+——-+——————-+———+———+——-+——+——-+ 1.idMy understanding is that the smooth execution of SQL is marked by the execution of SQL […]

  • MySQL optimized index


    1. How MySQL uses indexes The index is used to quickly find rows with specific column values. If there is no index, MySQL must start from the first row, and then traverse the entire table to find the relevant rows. The larger the watch, the more it costs. If there are indexes of related columns […]

  • Magic SQL having → easily despised protagonist


    a moment of fun One day, the landlord and the little boy next door took the elevator together, and a tall beauty came in halfway, holding a snow-white poodle The little boy looked at the snow-white poodle and liked it very much. He said, “aunt, can I touch this dog?”? Beauty: call elder sister The […]

  • Detailed explanation of SQL select query statement usage


    SQL overview The full name of SQL is structured query language, which was first proposed in the 1970s and is widely used to access and manage relational databases (such as oracle/mysql). Based on the comprehensive and powerful functions and simple and easy-to-use features of SQL, it has been thriving for 50 years; Many big data […]

  • Hive of big data technology Chapter 6 basic query


    Chapter 6 inquiry https://cwiki.apache.org/confluence/display/Hive/LanguageManual+Select Query statement syntax: SELECT [ALL | DISTINCT] select_expr, select_expr, … FROM table_reference [WHERE where_condition] [GROUP BY col_list] [ORDER BY col_list] [CLUSTER BY col_list | [DISTRIBUTE BY col_list] [SORT BY col_list] ] [LIMIT number] 6.1 basic query (select… From) 6.1.1 full table and specific column query 0) data preparation (0) raw data […]

  • Oracle Foundation (IX): execution order of sorting, aggregate function, grouping and query statements


    1、 Sort (1) Sort result set ORDER BYClause is used to sort the result set by the specified field. There are two ways to sort:ASC (ascending): if you do not write, the default is ascending, from small to large.Desc (descending): from large to small, it needs to be specified separately. ORDER BY Clause must be […]

  • MySQL schema Overview – > query execution process – > sql parsing sequence


    preface I always want to know how an SQL statement is executed, and what is the order of its execution. Then I check and summarize the data of all parties, and I have the following blog post. This article will discuss the knowledge from the overall MySQL Architecture – > query execution process – > […]

  • Introduction to AI knowledge


    Introduction to artificial intelligence — artificial intelligence neural and network, (only the key points are recorded) Artificial intelligence (AI) is a new technology science that researches and develops theories, methods, technologies and application systems for simulating, extending and expanding human intelligence. knowledge representation ✨ Basic concepts of knowledge and knowledge representation knowledgeThe information structure formed […]

  • Class field in typescript


    Examples are as follows: The TSC compilation option related to this knowledge point: strictpropertyinitialization The strictpropertyinitialization setting controls whether class fields need to be initialized in the constructor. Correct practice: class GoodGreeter { name: string; constructor() { this.name = “hello”; } } Note that this field needs to be initialized in the constructor itself. Typescript […]

  • Chapter 45 SQL command from (I)


    Chapter 45 SQL command from (I) A select clause that specifies one or more tables to query. outline SELECT … FROM [optimize-option] table-ref [[AS] t-alias][,table-ref [[AS] t-alias]][,…] parameter optimize-optioN – optional – specifies a single keyword or a series of keywords separated by spaces for query optimization options (optimizer tips). The following keywords are supported:%ALLINDEX、%FIRSTTABLE […]

  • [data analysis tool] basic SQL syntax


    01. Write in front SQL is one of the necessary skills for data analysts. Whether it is a junior analyst or a senior analyst, SQL has become a necessary option in the recruitment conditions of major companies. Why is SQL so important for data analysts? Before answering this question, let’s understand the following questions. First […]

  • MySQL basic syntax


    Concept of SQL What is SQL: Structured Query Language SQL function: through SQL statements, we can easily operate the database, tables and data in the database. SQL is a standard that all database management systems need to follow. Different database manufacturers support SQL statements, but they all have unique contents. Classification of SQL language: DDL […]