SQL查询测试2

SET NAMES utf8mb4;

SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------

-- Table structure for customers

-- ----------------------------

DROP TABLE IF EXISTS `customers`;

CREATE TABLE `customers`  (

  `customer_id` int(0) NOT NULL AUTO_INCREMENT,

  `name` varchar(100) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_bin NOT NULL,

  `email` varchar(100) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_bin NULL DEFAULT NULL,

  `phone` varchar(15) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_bin NULL DEFAULT NULL,

  PRIMARY KEY (`customer_id`) USING BTREE,

  UNIQUE INDEX `email`(`email`) USING BTREE

) ENGINE = InnoDB AUTO_INCREMENT = 5 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_0900_bin ROW_FORMAT = Dynamic;

-- ----------------------------

-- Records of customers

-- ----------------------------

INSERT INTO `customers` VALUES (1, 'John Doe', 'john@example.com', '1234567890');

INSERT INTO `customers` VALUES (2, 'Jane Smith', 'jane@example.com', '0987654321');

INSERT INTO `customers` VALUES (5, 'Blau Hoss', 'Blau@example.com', '11111111');

-- ----------------------------

-- Table structure for employees

-- ----------------------------

DROP TABLE IF EXISTS `employees`;

CREATE TABLE `employees`  (

  `id` int(0) NULL DEFAULT NULL,

  `name` varchar(30) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_bin NULL DEFAULT NULL,

  `department` varchar(30) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_bin NULL DEFAULT NULL,

  `salary` int(0) NULL DEFAULT NULL,

  `manager_id` int(0) NULL DEFAULT NULL

) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_0900_bin ROW_FORMAT = Dynamic;

-- ----------------------------

-- Records of employees

-- ----------------------------

INSERT INTO `employees` VALUES (1, 'Aditi', 'HR', 30000, 5);

INSERT INTO `employees` VALUES (2, 'Rahul', 'IT', 50000, 6);

INSERT INTO `employees` VALUES (3, 'Neha', 'IT', 60000, 6);

INSERT INTO `employees` VALUES (4, 'Aman', 'Sales', 40000, 7);

INSERT INTO `employees` VALUES (5, 'Kiran', 'HR', 70000, 0);

INSERT INTO `employees` VALUES (6, 'Mohit', 'IT', 80000, 0);

INSERT INTO `employees` VALUES (7, 'Suresh', 'Sales', 65000, 0);

INSERT INTO `employees` VALUES (8, 'Pooja', 'HR', 30000, 5);

SET FOREIGN_KEY_CHECKS = 1;

此查询的输出结果是什么？
WITH numbers AS ( SELECT 10 AS value UNION SELECT 20 )
SELECT SUM(value) FROM numbers;

此查询将返回什么结果？

SELECT name FROM customers
WHERE NOT EXISTS (
SELECT * FROM orders WHERE customers.customer_id = orders.customer_id
);

此查询将返回什么结果？
SELECT salary FROM (
SELECT salary, DENSE_RANK() OVER (ORDER BY salary DESC) r
FROM employees
) t WHERE r = 2;