Question

我正在学习一些基本的 SQL 查询技能，并处理日期。我成功获得了以下内容。

SELECT
    [FirstName],
    [LastName],
    [JobTitle],
    [JoiningDate]
    DATEDIFF(day, StartDate, '2021/03/20') AS DateDifference
FROM 
    [GenericITCompany].[dbo].[Employees]
WHERE 
    JobTitle = 'UnityDeveloper';

这将返回 Unity 团队的每位开发人员在公司工作的天数。

现在，我想知道谁在公司待的时间最长。

我试过了：

SELECT
    [FirstName],
    [LastName],
    [JobTitle],
    [JoiningDate]
    DATEDIFF(day, StartDate, '2021/03/20') AS DateDifference
    MAX(DateDifference) AS LongestServingEmployee
FROM 
    [GenericITCompany].[dbo].[Employees]
WHERE 
    JobTitle = 'UnityDeveloper';

那是行不通的。我可能遗漏了一些非常明显的东西。

注1：我了解Max的基本用法。例如，

SELECT 
    MAX(StartDate) AS MaximumStartDate
FROM
    [GenericITCompany].[dbo].[Employees]

但是，我主要面临将 MAX 应用于查询生成表的挑战。我相信这是我的主要问题。

注2：我查看了一些存在类似问题的现有问题

Fetch the row which has the Max value for a column

How can I SELECT rows with MAX(Column value), DISTINCT by another column in SQL?

但我无法从他们那里理解。

Answer 1

您可以像以下子查询一样使用 MIN 函数来获取最小日期而不是日期差异。

SELECT
[FirstName],
[LastName],
[JobTitle],
[JoiningDate]
DATEDIFF(day, StartDate, '2021/03/20') AS DateDifference
FROM [GenericITCompany].[dbo].[Employees]
WHERE JobTitle='UnityDeveloper'
-- Getting the smallest StartDate
AND StartDate = (
  SELECT MIN(StartDate) AS MaximumStartDate
  FROM [GenericITCompany].[dbo].[Employees]
)

Answer 2

不能在同一个查询中使用 DateDifference 列名


SELECT
[FirstName],
[LastName],
[JobTitle],
[JoiningDate],
DATEDIFF(day, StartDate, '2021/03/20') AS DateDifference
INTO #Employees
FROM [GenericITCompany].[dbo].[Employees]
WHERE JobTitle='UnityDeveloper';

Select  *
From #Employees where DateDiffernce = (Select Max(DateDiffernce) From #Employees)

Answer 3

我不清楚您希望看到的结果是什么，您是否只想看到服务日期最长的员工（可能有关系？）

或者每行天数最多的所有员工 - 这似乎是您的查询试图执行的操作，在这种情况下，您可以执行以下操作，并按 LongestServiceEmployee 排序。我还用 from tensorflow.keras.preprocessing import image from tensorflow.keras.applications.vgg16 import VGG16, preprocess_input from tensorflow.keras.models import Model import numpy as np from PIL import Image #from feature_extractor import FeatureExtractor from datetime import datetime from flask import Flask, request, render_template from pathlib import Path from keras.optimizers import Adam from tensorflow.keras.layers import Dropout, Dense, Activation, Flatten class FeatureExtractor: def __init__(self): input_shape = (224, 224, 3) base_model = VGG16(weights='imagenet', include_top=False, input_shape=input_shape) for layer in base_model.layers: layer.trainable = False last = base_model.layers[-1].output x = Flatten()(last) x = Dense(1000, activation='relu', name='fc1')(x) x = Dropout(0.3)(x) x = Dense(10, activation='softmax', name='predictions')(x) model = Model(base_model.input, x) model.compile(optimizer=Adam(lr=0.001), loss = 'categorical_crossentropy',metrics=['accuracy']) self.model = Model(inputs=base_model.input, outputs=base_model.layers[-1].output) def extract(self, img): """ Extract a deep feature from an input image Args: img: from PIL.Image.open(path) or tensorflow.keras.preprocessing.image.load_img(path) Returns: feature (np.ndarray): deep feature with the shape=(4096, ) """ img = img.resize((224, 224)) # VGG must take a 224x224 img as an input img = img.convert('RGB') # Make sure img is color x = image.img_to_array(img) # To np.array. Height x Width x Channel. dtype=float32 x = np.expand_dims(x, axis=0) # (H, W, C)->(1, H, W, C), where the first elem is the number of img x = preprocess_input(x) # Subtracting avg values for each pixel feature = self.model.predict(x)[0] # (1, 4096) -> (4096, ) return feature / np.linalg.norm(feature) # Normalize path = "/home/virtuag/www/storage/searchSCB.jpg" img = Image.open(path) app = Flask(__name__) fe = FeatureExtractor() features = [] img_paths = [] for feature_path in Path("/home/virtuag/www/storage/images_article").glob("*.npy"): features.append(np.load(feature_path)) img_paths.append(Path("/home/virtuag/www/storage/images_article") / (feature_path.stem + ".jpg")) features = np.array(features) query = fe.extract(img) dists = np.linalg.norm(features-query, axis=1) ids = np.argsort(dists)[:30] scores = [img_paths[id] for id in ids] 代替了您的硬编码日期

GETDATE()

如果您只想找到具有最大值的员工，则将查询包装在外部 SELECT [FirstName], [LastName], [JobTitle], [JoiningDate] DATEDIFF(day, createdate, GETDATE()) AS DateDifference MAX(day, createdate, GETDATE()) over() AS LongestServingEmployee FROM [dbo].[Employees] WHERE JobTitle = 'UnityDeveloper' order by LongestServingEmployee desc; 中并进行过滤：

select

计算日期差异后选择最大日期差异

3 个答案: