1. Concept: Java database connectivity, Java database connection, Java language operation database
JDBC essence: it is actually a set of rules defined by the official (sun company) to operate all relational databases, that is, interfaces. Each database manufacturer implements this interface and provides database driver jar package. We can use ...
Added by Pehape on Mon, 21 Feb 2022 06:58:21 +0200
MySQL has become the first choice for most companies with low-cost and rich Internet resources. Although the performance is excellent, how to better use the so-called "good horse with good saddle" has become a compulsory course for development engineers. We often see requirements such as "proficient in MySQL", &q ...
Slow query and slow log
Query and start of slow log
Query whether the slow log is enabled
mysql> show variables like 'slow_query_log';
| Variable_name | Value |
| slow_query_log | OFF |
1 row in set (0.00 sec)
Enable slow query log
# It is convenien ...
1, Meaning of subquery
Meaning: select statements that appear in other statements are called subqueries or intra queries; The external query statement is called primary query or external query Classification:
According to the location of sub query: After select: only scalar subqueries are supported After from: only table subqueries are ...
Added by sullyman on Sun, 20 Feb 2022 19:44:44 +0200
The ECS that Alibaba cloud bought for 95 yuan is about to expire. It needs more than 1000 to renew. I thought there was nothing important on the server, so I took advantage of Tencent cloud discount and spent 288 to buy a three-year CVM. CVM is a cloud virtual machine. Let's call it a server for the time being.
1 core 2G, it seems tha ...
Connection query (this article only introduces the connection query of SQL 92 standard in detail)
1, Meaning and classification of connection query
Also known as multi table query, join query will be used when the query fields come from multiple tables Cartesian product phenomenon:
Suppose that table 1 has m rows, table 2 has n ...
Added by deljhp on Sun, 20 Feb 2022 10:54:30 +0200
Database structure diagram
1. Retrieve data
1.1 select database
1.2 select a single column
select prod_name from Products;
| prod_name |
| Fish bean bag toy |
| Bird bean bag toy |
| Rabbit bean bag toy |
| 8 inch teddy bear |
| 12 inch teddy bear |
Added by sc00tz on Sun, 20 Feb 2022 09:13:52 +0200
1.1 what are the sub warehouse and sub table
Xiao Ming is a developer of a start-up e-commerce platform. He is responsible for the function development of the seller module, The related businesses of shops and commodities are involved, and the design is as follows Database:
Store information and geographical area inform ...
Added by new_programmer on Sun, 20 Feb 2022 07:09:11 +0200
– one Prepare data
Four tables are known as follows: Student form: student (student number, student name, date of birth, gender) Transcript: score (student number, course number, grade) Course schedule: Course (course number, course name, teacher number) Teacher form: teacher (teacher number, teacher name) Write the corresponding SQL sta ...
Added by corbin on Sun, 20 Feb 2022 03:58:46 +0200