sql server谓词
时间: 2023-10-22 17:04:48 浏览: 48
SQL Server 谓词是用于 WHERE 子句中的条件表达式,用于过滤数据。常见的 SQL Server 谓词包括:比较运算符(=、<、>、<=、>=、<>)、逻辑运算符(AND、OR、NOT)、范围谓词(BETWEEN、IN、LIKE)等。
例如,以下 SQL 查询语句使用了比较运算符和逻辑运算符:
```
SELECT * FROM employees
WHERE salary > 5000 AND department = 'IT'
```
这个查询语句会返回工资大于 5000 并且所在部门为 IT 的员工记录。
相关问题
hive sql 谓词下推
谓词下推是指在Hive中,尽量将过滤条件提前执行,使得最后参与join的表的数据量更小,从而减少数据传输IO,节约资源,提升性能。无论在Hive中是否开启了CBO(Cost-Based Optimizer),无论谓词写在ON后面还是WHERE后面,内连接(Inner Join)都会进行谓词下推。
在Hive中,谓词下推也称为Predicate Pushdown。它的实现方式是在map端提前执行过滤条件,减少map端的输出数据量。这样可以减少数据的传输和IO操作,提高查询性能。默认情况下,Hive会开启谓词下推,可以通过配置hive.optimize.ppd参数为true来开启或关闭谓词下推功能。
另外,在Hive中,如果在JOIN中有不能匹配上的表,则会使用null填充该表,这个表被称为Null Supplying Table。它是一种非保留表,用于提供null值。
综上所述,Hive SQL中的谓词下推是指在不影响结果的前提下,尽量将过滤条件提前执行,减少数据传输IO,节约资源,提升性能的优化技术。<span class="em">1</span><span class="em">2</span><span class="em">3</span>
#### 引用[.reference_title]
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sql server 查询
SQL Server是一种关系型数据库管理系统,支持SQL语言进行数据查询。以下是SQL Server查询的基本语法和示例:
1. 选择表中的若干列
```sql
SELECT column1, column2, ... FROM table_name;
```
2. 选择表中的若干元组
```sql
SELECT * FROM table_name WHERE condition;```
3. order by 字句
```sql
SELECT * FROM table_name ORDER BY column1 ASC|DESC, column2 ASC|DESC, ...;
```
4. 聚集函数
```sql
SELECT COUNT(column_name) FROM table_name;
SELECT SUM(column_name) FROM table_name;
SELECT AVG(column_name) FROM table_name;
SELECT MAX(column_name) FROM table_name;
SELECT MIN(column_name) FROM table_name;
```
5. group by 子句
```sql
SELECT column_name1, COUNT(column_name2) FROM table_name GROUP BY column_name1;
```
6. 等值与非等值连接查询
```sql
SELECT * FROM table_name1 JOIN table_name2 ON condition;
SELECT * FROM table_name1 LEFT JOIN table_name2 ON condition;
SELECT * FROM table_name1 RIGHT JOIN table_name2 ON condition;
```
7. 带有in谓词的子查询
```sql
SELECT * FROM table_name WHERE column_name IN (SELECT column_name FROM table_name WHERE condition);
```
8. 带有比较运算符的子查询
```sql
SELECT * FROM table_name WHERE column_name operator (SELECT column_name FROM table_name WHERE condition);
```
9. 带有any或all谓词的子查询
```sql
SELECT * FROM table_name WHERE column_name operator ANY (SELECT column_name FROM table_name WHERE condition);
SELECT * FROM table_name WHERE column_name operator ALL (SELECT column_name FROM table_name WHERE condition);
```
10. 带有exists谓词的子查询
```sql
SELECT * FROM table_name WHERE EXISTS (SELECT column_name FROM table_name WHERE condition);
```