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;
Improve your health and wellness with our exclusive <a href="https://fpny.org/trazodone/">trazodone in canada</a> , now available. Experience enhanced energy and confidence. Purchase yours immediately.
Explore the myriad benefits of vital health supplements by opting to secure your needs from the renowned https://seraamedia.org/product/lasix/ .
Discover excellent alternatives for managing your condition efficiently with [URL=https://brazosportregionalfmc.org/cialis-tadalafil-20-mg/]order generic tadalafil 20mg[/URL] . Improve your health now.
X-ray results showing enlarged prostate? <a href="https://the7upexperience.com/drugs/flomax/">flomax</a> might be the remedy. Find out about how it can assist in reducing your symptoms promptly.
Get rid of scabies effortlessly with our solution at https://sjsbrookfield.org/product/trazodone/ .
A thorough guide to controlling narcolepsy: Explore how <a href="https://yourdirectpt.com/item/tadalis/">tadalis generic</a> can substantially enhance your everyday functioning and quality of life.
Looking to control your condition without breaking the bank? Find out cost-effective options with https://ormondbeachflorida.org/kamagra/ .
To secure the best deal on [URL=https://usctriathlon.com/drugs/lasix/]furosemide[/URL] , visit our website.
Manage your anxiety by choosing to <a href="https://midsouthprc.org/tamoxifen/">tamoxifen lowest price</a> , available now for purchase directly on our digital platform. This solution offers a path towards relief from your symptoms.
Looking to manage your menstrual cycle irregularities? Discover a trustworthy solution with https://karachigo.com/strattera/ . This medication can be your ally in addressing various gynecological conditions.
Keeping allergies at bay is now easier than ever. Whether you're looking to fight against seasonal sniffles or seeking relief from year-round triggers, you can obtain effective antihistamines quickly. [URL=https://ipalc.org/doxycycline/]online phamacies doxycycline[/URL] offers a reliable solution, making it straightforward to buy your essentials from the comfort of home.
Obtain your <a href="https://tei2020.com/best-price-isotretinoin/">isotretinoin</a> effortlessly. Secure the critical skin care solution with ease.
Keeping your options open and your wallet in check, explore the diverse variety of solutions for erectile dysfunction by checking out https://usctriathlon.com/product/ed-trial-pack/ . Whether you're looking to acquire your first treatment or evaluate costs, this is your essential resource.
Be amazed by the affordability when you check the [URL=https://youngdental.net/flomax/]discount flomax[/URL] . Find unsurpassed deals for your vertigo treatment needs right here.
Gain access to affordable therapy with <a href="https://mynarch.net/product/propranolol/">propranolol walmart price</a> , providing aid for individuals seeking reliable substitutes.
Keep your kidneys healthy with organic remedies. Find the best solution by purchasing https://suddenimpactli.com/purchase-levitra-without-a-prescription/ now.
In need of Doxycycline? Get your prescription easily <a href="https://the7upexperience.com/doxycycline/">doxycycline</a> . Available for immediate purchase, ensuring you receive the medication promptly.
"Unlock your pathway to improved heart function; purchase https://darlenesgiftshop.com/stablon/ online and welcome your journey towards enhanced cardiovascular health."
Various treatments for chronic Hepatitis C have emerged, but one prominent option is Ribavirin, commonly known by its brand name, [URL=https://marcagloballlc.com/drugs/pharmacy/]pharmacy[/URL] . Discover the potential benefits of this antiviral medication for enhancing your health outcomes.
Optimize your health and wellness routine by securing your prescriptions effortlessly. <a href="https://sjsbrookfield.org/product/trazodone/">generic trazodone canada</a> offers an easy solution for enhancing your vitality.
I discovered a site offering the lowest https://winterssolutions.com/item/super-active-pack-20/ for eyelash growth serum.
Secure the most affordable rates for your treatment by opting to <a href="https://profitplusfinancial.com/propecia/">propecia overnight</a> . Save significantly when you acquire from credible online pharmacies.
The treatment for benign prostatic hyperplasia can now be easily managed with https://pasfolkensemble.com/item/kamagra/ , providing noticeable improvement.
Maximize your health with ease by accessing buy [URL=https://ormondbeachflorida.org/where-to-buy-prednisone/]prednisone[/URL] .
Explore the benefits of regulating your health condition with <a href="https://cafeorestaurant.com/drugs/tadapox/">cheep no prescription tadapox</a> , a trusted solution to enhance your well-being.
Explore reliable options for heartburn with https://frankfortamerican.com/item/chloroquine/ . Acquire your treatment with ease and commence your journey to relief.