Questions about SQL database basics

This is a generality issue, it may be that some functions are supported by access, but not by SQL Server.

Or the most famous example is that SQL does not support rownum, such as select * from student where rownum < = 10;

Wait a minute, the source code of your program is for access.

It's a pain in the ass to fix that.

Also, as you mentioned in your second question, you may not be linked to the SQL database ......The system does not need to be configured, as long as you install it, but there must be a link configuration in the program. Generally, it contains database names, usernames, passwords, etc., and the format of each development tool is different.

To add: after looking at your source**, it is recommended to use sql2000 to try.

I feel that 2000 is a little more reliable.

1.First of all, make sure that you have installed SQL Server on your machine, such as SQL Server Personal Edition (this version can be run under Win XP), etc.

2.Modify database connection.

Don't have SQL Server installed on your computer? If you want to call SQL database. The system must be equipped with a SQL server. Now we all use sql2005

I started the server of 05 and the tray doesn't seem to have an icon. Do you manually start right? This service should be started: SQL Server (MSSQLSarker).

1. SQL Overview.

1. What is SQL?

SQL (Structured Query Language) is the language used to operate on relational databases. It can be applied to all relational databases, such as MySQL, Oracle, SQL Server, etc.

The SQL standards (ANSI ISO) are: SQL-92: SQL language standard published in 1992; sql:

1999: SQL language tags released in 1999; sql:2003：

SQL language tags released in 2003; These standards are just like the JDK versions, and there are always some syntax changes in the new version. Different standards have been implemented in databases from different periods.

While SQL can be used in all relational databases, many databases also have some syntax that follows the standard and what we can call "dialects". For example, the limit statement in MySQL is a dialect unique to MySQL and is not supported by other databases! Of course, either Oracle or SQL Server have their own dialects.

1.What is a database If we take it in a narrow sense, it is a warehouse that stores data. If we take it broadly ,..

2.Why do you need a database?

3.Almost all applications require a database in the background. For example: banks. Sheet.

Three, Li Si....Wouldn't we need a vault to store this information when we put money in the bank,..?

4.The database stores data and takes up little space, and it is easy to persist in the chain family. For example: the same data, if you let it directly.

5.Does all the software use SQL Server?Most applications may use data.

The database, simply put, can be regarded as an electronic file cabinet - a place to store electronic documents, and users can add, intercept, update, delete and other operations on the data in the file.

Characteristics of the database system.

The database system realizes the structuring of the overall data, which is one of the most important characteristics of the database. The "holistic" structure here means that the data in the database is no longer just for a single application, but for the whole organization; Not only is the data internally structured, but it is also structured as a whole, and there are connections between the data. Because the data is holistically oriented, the data can be shared and used by multiple users and applications, which can greatly reduce data redundancy, save storage space, and avoid incompatibilities and inconsistencies between data.

High data independence.

Data independence includes both the physical and logical independence of the data. Physical independence refers to how the data is stored in the database on disk is managed by the DBMS, and the user program does not need to understand, and the application only needs to deal with the logical structure of the data, so that when the physical storage structure of the data changes, the user's program does not need to change. Logical independence means that the logical structure of the user's application and the database is independent of each other, that is, if the logical structure of the data changes, the user's program can not change.

Let's take a look at what is a database? How do I learn a database?

1. The data structure is cultivated, and the data is connected with each other and is oriented to the whole system.

2. High data sharing, low redundancy, and easy expansion.

3. High data independence.

So in order to get better applicability and cost performance, we will choose other types of databases, and what are the most commonly used databases?

Generally, in the field of automation, there are three types of databases that we commonly use: Microsoft SQL Server, Oracle and MySQL.

These three are relational databases, of course, some people say why not mention Microsoft Access, you can search for the difference between it and the other three to know why, of course, for relational databases, there are db2, postgresql, etc. For non-relational databases, it is currently less used in the field of automation.

In the field of industrial automation, we often come into contact with host computer software, such as our commonly used wincc, ifix, factorytalk view, wiscada, etc., which will involve the storage of historical data, for historical trends, historical alarms, report display, many companies have launched their own real-time database software, such as GE's Proficy Historian, Wonderware Historian, Honeywell's PHD, etc., but this kind of software is high ** and not very versatile, so it is not suitable for IT personnel.

Data is centrally controlled, and data is managed in a decentralized state, with different users or the same user having no relationship between their files in different processes. The database allows for centralized control and management of data, and a data model that represents the organization of various data and the connections between them.

Databases

Definition. A database is a repository of data. It has a large storage space and can store millions, tens of millions, and hundreds of millions of pieces of data.

However, the database does not store data at random, and there are certain rules, otherwise the query efficiency will be very low. Today's world is an internet world full of data, and it is full of data. That is, this Internet world is the world of data.

There are many types of data, such as travel records, consumption records, web pages browsed, messages sent, and so on. In addition to text-type data, images, **, and sounds are all data.

A database is a computer software system that stores and manages data in a data structure. The concept of a database actually has two meanings:

1) The database is an entity, it is a "warehouse" that can reasonably keep data, and users store the transaction data to be managed in the "warehouse", and the two concepts of "data" and "library" are combined to form a database.

2) Database is a new method and technology of data management, which can organize data more appropriately, maintain data more conveniently, control data more closely and use data more efficiently.

Development status. In the history of database development, the database has experienced various stages of development, such as hierarchical database, mesh database and relational database, and the rapid development of database technology in all aspects. Since the 80s, almost all new database products from database vendors have supported relational databases, and even some non-relational database products have almost all interfaces that support relational databases.

This is mainly due to the fact that traditional relational databases can better solve the problem of managing and storing relational data. With the development of cloud computing and the advent of the era of big data, relational databases are increasingly unable to meet the needs, which is mainly due to the fact that more and more semi-relational and non-relational data need to be stored and managed by databases. They place more emphasis on the high concurrency of database data to read, write, and store big data, and this type of database is generally referred to as NoSQL (not only SQL) databases. However, traditional relational databases still maintain strong vitality in some traditional fields.

** Encyclopedia.

In the first question, group by is not used

The second question is that the where condition cannot be used as an aggregate function.