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- What is the relationship between tables and databases in access?
- The relationship between tables and databases in access is: a database can contain multiple tables. Access is a database management system that manages information through various database objects; these objects are all stored in the same database file. A database can have multiple tables, and tables can have relationships or be independent of each other.
- SQL 21998 2020-07-06 16:57:37
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- What is the difference between sql left join and right join?
- The difference between sql left connection and right connection is that the left connection will read all the data in the left data table, even if the right data table does not have corresponding data; while the right connection will read all the data in the right data table, even if the left data table does not have corresponding data.
- SQL 8644 2020-07-06 16:17:57
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- What are the three most commonly used Chinese databases?
- The three most commonly used Chinese databases are: "MySQL", "SQL Server", and "Oracle". "MySQL" has the characteristics of small size and low cost; "SQL Server" has the characteristics of ease of use and friendly user interface; "Oracle" has the characteristics of good system portability, fast and flexible.
- SQL 25457 2020-07-06 15:43:23
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- How many primary keys are there in a single data table of the database?
- There can be only one primary key in a single data table of the database. A database primary key refers to a column or a combination of multiple columns, whose value can uniquely identify each row in the table, through which the entity integrity of the table can be enforced; the primary key is mainly used to associate foreign keys with other tables, as well as this Modification and deletion of records.
- SQL 14933 2020-07-06 15:20:00
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- What database language does sql belong to?
- SQL is a relational database language. Relational database language is a database language with multiple functions such as data manipulation and data definition. This language has interactive characteristics, can provide users with great convenience, and can use SQL language to improve the work quality and performance of computer application systems. efficiency.
- SQL 16350 2020-07-06 16:18:34
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- What are the applications of database management systems?
- Applications of database management systems: 1. Improve the security of data encryption systems; 2. Improve information storage efficiency; 3. Improve data backup and recovery; 4. Enhance multimedia management. The database management system provides unified management and control of the database to ensure the security and integrity of the database.
- SQL 9231 2020-07-04 10:10:57
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- The role of database index
- The biggest function of a database index is to speed up queries. It can fundamentally reduce the number of record rows that need to be scanned. A database index is the data structure of the database. Furthermore, the data structure stores all the values of a certain column in a table. , that is to say, the index is created based on a certain column in the data table.
- SQL 8745 2020-07-03 17:19:44
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- What are the SQL statements to delete fields in a table?
- SQL statements to delete fields in the table include: 1. Delete columns without default values [alter table Test drop COLUMN BazaarType]; 2. Delete columns with default values [alter table Test DROP COLUMN BazaarType].
- SQL 15498 2020-07-03 16:20:48
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- What is the sql statement to modify the table name?
- The SQL statement to modify the table name is the "ALTER TABLE" statement; the basic syntax is: "ALTER TABLE <table name> [modification options]". The "ALTER TABLE" statement is used to modify the design of an existing table; columns can be added, modified, or deleted from an existing table.
- SQL 32976 2020-07-03 16:00:45
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- What are the SQL date format conversion functions?
- The sql date format conversion functions are: 1. [Select CONVERT(GETDATE(), 23)] formats the current date into [year-month-day] format; 2. [Select CONVERT(GETDATE(), 8] formats the current time Format it into [hour:minute:second] format.
- SQL 14165 2020-07-03 11:33:47
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- What are the characteristics of nosql database
- The characteristics of nosql database are: 1. Easy to expand; 2. Large data volume and high performance; 3. Flexible data model; 4. High availability. There are many types of NoSQL databases, but they all have a common feature, that is, they all remove the relational characteristics of relational databases.
- SQL 35252 2020-07-03 11:05:00
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- What are the relational operations of the database?
- The relational operations of the database include: 1. Selection operation, which selects all tuples that meet given conditions from a relation R; 2. Projection operation, which selects certain specified attributes from all attributes of a relation R; 3. Connection operation, It selects tuples that meet certain conditions between attributes from the generalized Cartesian product of two relationships to form a new relationship.
- SQL 30117 2020-07-03 09:28:03
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- What is the meaning of group by in sql query
- The meaning of group by in SQL query is to combine the total function to group the result set according to one or more columns. Its function is to divide a data set into several small areas through certain rules, and then target several A small area for data processing.
- SQL 23020 2020-07-02 17:19:54
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- What kind of database is hbase?
- HBase is a distributed, column-oriented open source database. The technology comes from the Google paper "Bigtable: A distributed storage system for structured data" written by Fay Chang.
- SQL 11014 2020-07-01 16:30:28
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- What are the methods for sql optimization?
- SQL optimization methods: 1. Try to avoid using [select *], useless fields will reduce query efficiency; 2. Avoid using in and not in, you can choose between and exists instead; 3. Avoid using or, you can choose union instead.
- SQL 9060 2020-07-01 15:55:05
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