MySQL 教程
/ EXISTS
EXISTS
MySQL EXISTS 运算符
EXISTS 运算符用于测试子查询中是否存在任何记录。
如果子查询返回一条或多条记录，EXISTS 运算符返回 TRUE。
EXISTS 语法
SELECT column_name(s)
FROM table_name
WHERE
EXISTS(SELECT column_name FROM table_name WHERE
condition);演示数据库
以下是 Northwind 示例数据库中 "Products" 产品表的选择:
| ProductID | ProductName | SupplierID | CategoryID | Unit | Price |
|---|---|---|---|---|---|
| 1 | Chais | 1 | 1 | 10 boxes x 20 bags | 18 |
| 2 | Chang | 1 | 1 | 24 - 12 oz bottles | 19 |
| 3 | Aniseed Syrup | 1 | 2 | 12 - 550 ml bottles | 10 |
| 4 | Chef Anton's Cajun Seasoning | 2 | 2 | 48 - 6 oz jars | 22 |
| 5 | Chef Anton's Gumbo Mix | 2 | 2 | 36 boxes | 21.35 |
从“供应商”表中选择:
| SupplierID | SupplierName | ContactName | Address | City | PostalCode | Country |
|---|---|---|---|---|---|---|
| 1 | Exotic Liquid | Charlotte Cooper | 49 Gilbert St. | London | EC1 4SD | UK |
| 2 | New Orleans Cajun Delights | Shelley Burke | P.O. Box 78934 | New Orleans | 70117 | USA |
| 3 | Grandma Kelly's Homestead | Regina Murphy | 707 Oxford Rd. | Ann Arbor | 48104 | USA |
| 4 | Tokyo Traders | Yoshi Nagase | 9-8 Sekimai Musashino-shi | Tokyo | 100 | Japan |
MySQL EXISTS 示例
以下 SQL 语句返回 TRUE 并列出产品价格低于 20 的供应商:
实例
SELECT SupplierNameFROM SuppliersWHERE EXISTS (SELECT ProductName FROM
Products WHERE Products.SupplierID = Suppliers.supplierID AND Price < 20);
»
以下 SQL 语句返回 TRUE 并列出产品价格等于 22 的供应商:
实例
SELECT SupplierNameFROM SuppliersWHERE EXISTS (SELECT ProductName FROM
Products WHERE Products.SupplierID = Suppliers.supplierID AND Price = 22);
»