EXISTS 运算符用于检查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，则 EXISTS 运算符将返回 TRUE。
SELECT column_name(s)
FROM table_name
WHERE EXISTS
(SELECT column_name FROM table_name WHERE condition)
以下是 Northwind 数据库的“Products”（“产品”）表中的示例：
| ProductID | ProductName | SupplierID | CategoryID | Unit | Price |
|---|---|---|---|---|---|
| 1 | Chais | 1 | 1 | 10 boxes x 20 bags | 18.00 |
| 2 | Chang | 1 | 1 | 24 - 12 oz bottles | 19.00 |
| 3 | Aniseed Syrup | 1 | 2 | 12 - 550 ml bottles | 10.00 |
| 4 | Chef Anton's Cajun Seasoning | 2 | 2 | 48 - 6 oz jars | 22.00 |
| 5 | Chef Anton's Gumbo Mix | 2 | 2 | 36 boxes | 21.35 |
以及表“Suppliers”（“供应商”）中的示例：
| SupplierID | SupplierName | ContactName | Address | City | PostalCode | Country | Phone |
|---|---|---|---|---|---|---|---|
| 1 | Exotic Liquid | Charlotte Cooper | 49 Gilbert St. | Londona | EC1 4SD | UK | (171) 555-2222 |
| 2 | New Orleans Cajun Delights | Shelley Burke | P.O. Box 78934 | New Orleans | 70117 | USA | (100) 555-4822 |
| 3 | Grandma Kelly's Homestead | Regina Murphy | 707 Oxford Rd. | Ann Arbor | 48104 | USA | (313) 555-5735 |
| 4 | Tokyo Traders | Yoshi Nagase | 9-8 Sekimai Musashino-shi | Tokyo | 100 | Japan | (03) 3555-5011 |
| 5 | Cooperativa de Quesos 'Las Cabras' | Antonio del Valle Saavedra | Calle del Rosal 4 | Oviedo | 33007 | Spain | (98) 598 76 54 |
以下 SQL 查询返回 TRUE 并包含产品价格低于 20 的供应商列表：
Run SQLSELECT SupplierName
FROM Suppliers
WHERE EXISTS (SELECT ProductName
FROM Products
WHERE Products.SupplierID = Suppliers.supplierID
AND Price < 20)
另一个 SQL 语句返回 TRUE 并包含产品价格等于 22 的供应商列表：
Run SQLSELECT SupplierName
FROM Suppliers
WHERE EXISTS (SELECT ProductName
FROM Products
WHERE Products.SupplierID = Suppliers.supplierID
AND Price = 22)