NOT IN (…) Number does not exist in a list 不在一个列表 col_name NOT IN (1, 3, 5)越是精确的条件筛选,会让结果更容易理解,同时因为条件在返回之前筛掉不必要的结果,SQL的运行速度也会快很多(想象一下你只是想看下最近有哪些大片上映,你没必要下载整个电影看一遍). ...
Constraints that are associated with external tables (Hadoop or HBase tables) are considered to be informational only; they are created by specifying the NOT ENFORCED clause. Aninformational constraintis a constraint attribute that can be used by the SQL compiler to improve access to data. Informat...
The two attributes tr name and email in the above table ‘reservation’ have unique values in the user’s table. Additionally, we used the CONSTRAINT keyword to name the UNIQUE restrictions. If we choose, we can utilise this name to eventually lift the UNIQUE restriction. We added a list o...
String does not exist in a list 不在列表 col_name NOT IN ("D", "E", "F") 小贴士?在字符串表达式中的字符串需要用引号 " 包含,如果不用引号,SQL会认为是一个属性列的名字,如:col_name = color 表示 col_name和color两个属性一样的行 col_name = "color" 表示 col_name 属性为字符串 "colo...
When designing an SQL database, there may be cases where you want to impose restrictions on what data can be added to certain columns in a table. SQL makes t…
CHECK Constraint: Makes sure that all values in a column satisfy certain criteria. Primary Key Constraint: Used to uniquely identify a row in the table. Foreign Key Constraint: Used to ensure referential integrity of the data.Each constraint is discussed in the following sections. Next: SQL NOT...
To have a proper database, you need to follow constraints that will create a table with certain conditions of SQL constraints. The SQL constraint is a rule used to maintain accuracy and integrity. There are some types of constraints in SQL that can be used based on the types of ...
The UNIQUE constraint in SQL Server ensures that you do not have duplicate values in a single column or combination of columns. These columns should be part of the UNIQUE constraints. SQL Server automatically creates anindexwhen UNIQUE constraints are defined. You can have only one unique value ...
First, let’s understand exactly what SQL is. SQL stands forStructured Query Language, in other words, it’s a specialized programming language that communicates with databases. SQL is the language used to create or manage databases and specifies all of the data held within. SQL statements are...
SQL constraints are used to specify rules for the data in a table. Constraints are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the table. If there is any violation between the constraint and the data action, the act...