Programmer Think

The reason for writing this article is that during the interview, the interviewer asked me what the difference is between a left join b and b right join a. I said there is no difference. The query results are the same, and the performance is the same. The interviewer said that the sequence of small tables and large tables has an impact on the p ...

1. Exits statement When MySQL treats the EXISTS clause, it still adopts the execution method of nested subquery. As shown in the following SQL statement: SELECT * FROM my_neighbor n LEFT JOIN my_neighbor_apply sra ON n.id = sra.neighbor_id AND sra.user_id = 'xxx' WHERE n.topic_status < 4 ...

1 SQL overview 1.1 what is SQL SQL (Structured Query Language) is a "structured query language", which is an operation language for relational databases. It can be applied to all relational databases, such as MySQL, Oracle, SQL Server, etc. SQL standards (ANSI/ISO) include: 1. SQL-92: SQL language standard issued in 1992; 2. SQ ...

1, Relationship between tables 1.1 one to one 1.2 one to many A class can have multiple students, which is called one to many relationship 1.3 many to many A teacher can lead multiple students, and a student can be taught by multiple teachers, so the relationship between the two tables is many to many. 2, Join table query 2.1 co ...

Multi table query, sub query, common function 1, Multi table query Multi table query: connect multiple tables through key fields with the same meaning in different tables to query the field information in different tables. Correspondence One to one: for example, the personnel information table and the personnel identity card correspon ...

Performance Monitoring Tools -- Connect MySQL Number of servers SHOW GLOBAL STATUS LIKE 'Connections'; -- MySQL Server online time SHOW GLOBAL STATUS LIKE 'Uptime'; -- Number of slow queries SHOW GLOBAL STATUS LIKE 'Slow_queries'; -- Select Number of rows returned by the query SHOW GLOBAL STATUS LIKE 'Innodb_rows_read'; -- implement INSERT Nu ...

1 View 1.1 what is a view A view is a virtual table, which is different from a direct operation data table. A view is created according to a SELECT statement. Therefore, when operating a view, a virtual table will be generated according to the SELECT statement that created the view, and then SQL operations will be performed on this virtual ta ...

Hive related configuration is required for Spark sql to read and write hive, so hive site is generally used The XML file is placed in the conf directory of spark. Code calls are simple. The key is the source code analysis process and how spark interacts with hive. 1. Code call Read hive code SparkSession sparkSession = SparkSession.builder() ...

Written test question 66: latest login date of each person of Niuke (I) Detailed description: please write an sql statement to query the last login date of each user, and press user_id ascending sort Problem solving ideas Group by userid and take the maximum login time source code select user_id, max(date) from login group by user_id order ...

Introduction: This paper will introduce how to do performance test based on TPCH data set in Hologres, and provide reference for test results to facilitate you to select product specifications. background information TPC-H (Business Intelligence Computing test) is a test set developed by the transaction processing performance Council (TPC) to ...