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Chapter 4 SQL aggregate function COUNT (2) jurisdiction To use COUNT(*), you must have table level SELECT permissions on the specified table. To use count (column name), you must have column level SELECT permission on the specified column or table level SELECT permission on the specified table. You can determine whether the current user h ...

1. Definition of MySQL master-slave synchronization Master-slave synchronization enables data to be copied from one database server to other servers. When copying data, one server acts as the master and the other servers act as slave serversBecause replication is asynchronous, the slave server does not need to be connected to the master server ...

Index optimization analysis 1, Reasons for SQL performance degradation Poor writing of query statements Index failure: the index is built but not used Too many join (design defect or unavoidable requirement) for association query Server tuning and setting of various parameters (buffer, number of threads, etc.) 2, Common common Join qu ...

multi-table query Table used in the example: emp table dept table Syntax: select * frome Table 1,Table 2, where condition; Tip: the condition of multi table query cannot be less than - 1 of the number of tables, otherwise the Cartesian product will appear, that is, the result is the number of rows in Table 1 * the number of rows ...

SQL statement operation_ MySQL core query 2 (multi table query) 1 foreign key constraints 1.1 foreign keys The foreign key refers to the field in the slave table corresponding to the primary key of the primary table (that is, there is A column in data table A, which points to the primary key of another data table B, and this column in da ...

When the program is running, the data is in memory. When the program terminates, it is usually necessary to save the data on disk. We learned earlier that writing data to a file is a way to save it to disk. However, when facing a large number of data, in order to facilitate us to save and find data or find specific data, the database needs to ...

catalogue 1, Environment construction 1. System planning 2. Configure DM ini 3. Configure dmmal ini 4. Configure dmmpp ctl 5. Run MPP 2, Problem solving 1. After starting two instances, start/ disql error  2. Common meter test 3. Temporary meter test 1, Environment construction Build a two node MPP cluster in linux environment ...

I remember in the previous article“ Logstash: API analysis using ELK stack ”Use Logstash to analyze the API of some indicators. In today's exercise, I will show how to use Logstash to analyze the service APIs of some log classes. We know that in many cases, we can quickly use some scripts to analyze the data of some service APIs. Th ...

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Task05: window functions, etc. - Tianchi Longzhu program SQL training camp Original tutorial link Recommended reference books: SQL basics tutorial Window function Window functions are also called OLAP functions. OLAP is short for online analytical processing, which means real-time analysis and processing of database data. Conventional ...