在SQL查询中，关键词SELECT、FROM、WHERE、GROUP BY、HAVING和ORDER BY的组合使用定义了数据的筛选、分组、排序等操作。理解这些关键字的执行顺序对于构建有效且高效的查询至关重要。以下是这些关键字的执行顺序详解及案例说明：
执行顺序详解
- FROM子句：
- 指定查询的数据源，既要从哪个表中获取数据。如果有多个表通过JOIN连接，那么会先计算这些表的笛卡尔积，然后根据JOIN条件和ON字句进行筛选。
- JOIN子句：
- 指定表之间的连接类型（如INNER JOIN、LEFT JOIN等）和连接条件。JOIN操作会在FROM子句之后进行，用于合并来自不同表的数据。
- WHERE子句：
- 在数据分组和聚合之前，根据指定的条件筛选记录。只有满足WHERE条件的记录才会被保留下来进行后续的操作。
- GROUP BY子句：
- 将筛选后的记录按照一个或多个列进行分组。分组后，后续的操作（如聚合函数）将作用于这些分组上。
- HAVING子句（如果有的话）：
- 对分组后的数据进行筛选。与WHERE子句不同，HAVING子句可以使用聚合函数。只有满足HAVING条件的分组才会被保留下来。
- SELECT子句：
- 选择需要返回的列或表达式。这包括聚合函数的结果、列的别名等。SELECT子句是在所有筛选和分组操作之后执行的。
- DISTINCT子句（如果有的话）：
- 对SELECT子句返回的结果进行去重。DISTINCT子句通常与SELECT子句一起使用，以确保返回的结果集中不包含重复的行。
- ORDER BY子句：
- 对查询结果进行排序。ORDER BY子句可以在SELECT子句之后指定一个或多个列作为排序的依据。排序可以是升序（ASC）或降序（DESC）。
- LIMIT子句（如果有的话）：
- 限制查询结果的行数。LIMIT字句通常用于分页查询，可以指定从结果集的哪一行开始返回，以及返回多少行。
案例分析
案例1：查询每个部门的员工数量，并按员工数量降序排列
SELECT department,COUNT(*) AS employee_count
FROM employees
GROUP BY department
ORDER BY employee_countDESC;
"Boost your peace of mind with a reliable human immunodeficiency virus testing solution. Buy the <a href="https://tei2020.com/drugs/cialis/">cialis coupon</a> today and confirm your status promptly from the privacy of your home."
Looking to improve your energy? Discover trusted options and buy https://tei2020.com/viagra-without-a-prescription/ —the solution to addressing intimacy issues.
Questing for the most affordable alternatives for managing pain? [URL=https://lasvegas-nightclubs.com/nizagara/]nizagara[/URL] presents a solution with significant savings.
Having unexpected expenses? Secure a <a href="https://yourdirectpt.com/pill/lamprene/">cheapest lamprene</a> without delay to take care of your financial needs.
Acquiring https://karachigo.com/strattera/ prescription could be effortless through web pharmacies, offering a hassle-free option.
Given the necessity for effective constipation relief, considering [URL=https://usctriathlon.com/finasteride-without-a-doctor/]finasteride[/URL] could be a wise decision.
Optimize your health with a variety of solutions by exploring <a href="https://transylvaniacare.org/product/ed-sample-pack-2/">ed sample pack 2 type medication</a> , offering an exclusive blend of options for your wellbeing.
With various online platforms available, it's never been easier to https://fpny.org/item/asthalin/ . Whether you're seeking to obtain pharmaceuticals easily, our platform offers a seamless and secure purchasing process.
Discover the best cost-effective solution for migraine relief by clicking <a href="https://the7upexperience.com/prednisone-40mg/">buy prednisone online cheap</a> . Secure your supply without overspending.
Unlock savings on allergy relief: https://yourdirectpt.com/item/tadalis/ online and experience relief from year-round allergies without breaking the bank.
Maximize your savings on antiretroviral therapy by clicking here: [URL=https://lasvegas-nightclubs.com/purchase-propecia-online/]propecia 1mg[/URL] . Find the best prices on the internet for effective HIV treatment options.
Yearning for cost-effective COVID-19 treatment options? Discover the solution with <a href="https://cafeorestaurant.com/drugs/secnidazole/">secnidazole</a> , where effectiveness meets affordability. Save on healthcare costs without compromising quality.
X-ray your options for antifungal treatment by exploring https://marcagloballlc.com/drugs/xenical/ . With a myriad of choices to acquire this vital treatment online, your path to recovery could be just a click away.
Venture into the world of affordable healthcare solutions and explore how you can buy [URL=https://usctriathlon.com/order-amoxicillin/]amoxicillin on internet[/URL] to control your health needs online.
Discover a diverse selection of health solutions with <a href="https://ormondbeachflorida.org/buying-lasix/">lasix homeopatic</a> , available for acquisition directly via the web.
The requirement for trustworthy and convenient access to medications has never been greater. Explore your options and purchase https://winterssolutions.com/parlodel/ now.
When seeking instant relief, explore our options to purchase high-quality treatments [URL=https://transylvaniacare.org/product/beloc/]beloc[/URL] .
Unlocking the secrets of innovative neurological treatments, <a href="https://lilliputsurgery.com/item/generic-prednisone-uk/">buy prednisone</a> stands as a pinnacle for study in neurodegenerative disorders.
Relieve your BPH symptoms today! Find our variety of https://goldenedit.com/drugs/flomax/ , specifically designed to boost your urination and overall health.
"Boost your peace of mind with a reliable HIV testing solution. Buy your [URL=https://goldenedit.com/drugs/flomax/]flomax[/URL] today and confirm your status quickly from the privacy of your home."
Boost your performance naturally with the powerful herbal blend found at <a href="https://bakuchiropractic.com/best-price-tadalafil/">tadalafil without dr prescription usa</a> . This topical solution, available for purchase via web, can boost vitality and stamina effectively.
In search of https://heavenlyhappyhour.com/women-pack-40/ ? Look no further for cost-effective solutions to your nasal symptoms.
Your quest for affordable the antimalarial drug ends here. Discover the current costs at [URL=https://thecultivarte.com/drugs/buying-hydroxychloroquine/]hydroxychloroquine[/URL] , your go-to destination for economical medicinal purchases.
Acquire
Zap vitality deficits and enhance wellness effortlessly by acquiring your https://fpny.org/item/retin-a/ to promote better health and longevity now.
Revitalize your vigor effortlessly with [URL=https://the7upexperience.com/drugs/viagra-overnight/]viagra[/URL] , now accessible to buy from the comfort of your home.
With competitive pharmacy rates, <a href="https://fontanellabenevento.com/product/lasipen/">buy lasipen w not prescription</a> offers an affordable solution to boost your fertility journey. Explore budget-friendly options now.
Obtaining https://bakuchiropractic.com/cheapest-retin-a-dosage-price/ has never been more simple, ensuring you acquire your medication without delay.
Secure effective relief from heartburn and acidity with [URL=https://myhealthincheck.com/lady-era/]lady era[/URL] . Whether you seek to purchase a remedy, it's obtainable for purchase online.