數(shù)據(jù)處理是專業(yè)人士經(jīng)常面對(duì)的問(wèn)題，尤其是在大型數(shù)據(jù)集的情況下。有效總結(jié)和分析數(shù)據(jù)非常重要，能從數(shù)據(jù)中獲取有價(jià)值的見(jiàn)解。SQL提供了一組強(qiáng)大的聚合函數(shù)，可以幫助數(shù)據(jù)科學(xué)家和數(shù)據(jù)分析師更好地處理和分析數(shù)據(jù)。

本文介紹10個(gè)實(shí)用的SQL聚合函數(shù)，并舉例說(shuō)明其在實(shí)際應(yīng)用中的使用方法，有助于讀者更好地理解SQL聚合函數(shù)的工作原理和應(yīng)用場(chǎng)景。

基本聚合函數(shù)

1. COUNT

用于計(jì)算表中的行數(shù)或列中的非空值數(shù)量。

SELECT COUNT(*) AS total_rows
FROM orders;

2. SUM

用于計(jì)算數(shù)值列中值的總和。

SELECT SUM(sales_amount) AS total_sales
FROM transactions;

3. AVG

用于計(jì)算數(shù)值列中值的平均值（平均數(shù)）。

SELECT AVG(price) AS average_price
FROM products;

4. MIN和MAX

可查找列中的最小值和最大值。

SELECT MIN(stock_quantity) AS min_quantity,
       MAX(stock_quantity) AS max_quantity
FROM inventory;

分組聚合函數(shù)

5. GROUP BY

按照一個(gè)或多個(gè)列對(duì)數(shù)據(jù)進(jìn)行分組，并對(duì)每個(gè)組應(yīng)用聚合函數(shù)。

SELECT category, AVG(price) AS avg_price
FROM products
GROUP BY category;

6. HAVING

根據(jù)聚合值對(duì)分組結(jié)果進(jìn)行過(guò)濾篩選，只有滿足指定條件的組才會(huì)被包含在結(jié)果集中。

SELECT category, AVG(price) AS avg_price
FROM products
GROUP BY category
HAVING AVG(price) > 100;

統(tǒng)計(jì)聚合函數(shù)

7. STDDEV和VARIANCE

用于計(jì)算數(shù)值列中的標(biāo)準(zhǔn)差和方差。

SELECT STDDEV(salary) AS salary_stddev,
       VARIANCE(salary) AS salary_variance
FROM employees;

8. CORR和COVAR

用于計(jì)算兩列之間的相關(guān)系數(shù)和協(xié)方差。

SELECT CORR(price, sales) AS price_sales_corr,
       COVAR(price, sales) AS price_sales_covar
FROM products;

與DISTINCT一起使用的聚合函數(shù)

9. COUNT(DISTINCT)

可計(jì)算列中不同值的數(shù)量。

SELECT COUNT(DISTINCT product_category) AS unique_categories
FROM products;

10. GROUP_CONCAT

可將多行中的值連接成單個(gè)字符串。

SELECT order_id, GROUP_CONCAT(product_name) AS ordered_products
FROM order_details
GROUP BY order_id;

這10個(gè)SQL聚合函數(shù)是數(shù)據(jù)分析師和數(shù)據(jù)科學(xué)家在處理大型數(shù)據(jù)集時(shí)非常有用的工具。掌握這些函數(shù)并了解如何使用函數(shù)可以幫助讀者更好地理解數(shù)據(jù)，從而做出明智的決策。