SQL查詢的執(zhí)行順序不是按照書寫順序進(jìn)行的，而是遵循特定的階段流程。首先，數(shù)據(jù)庫(kù)從FROM和JOIN操作開始獲取數(shù)據(jù)源；其次，通過(guò)WHERE子句過(guò)濾行；隨後，使用GROUP BY將數(shù)據(jù)分組；接著，HAVING篩選符合條件的分組；然後，SELECT指定返回的字段；最後，ORDER BY對(duì)結(jié)果排序。例如：1. FROM/JOINs確定涉及的表並拉取數(shù)據(jù)；2. WHERE過(guò)濾匹配的行；3. GROUP BY按列組織成組；4. HAVING篩選聚合後的分組；5. SELECT選擇輸出字段並定義別名；6. ORDER BY根據(jù)指定列排序結(jié)果。掌握該順序有助於編寫高效查詢。

When a SQL query runs, it doesn't execute in the same order you write it. Knowing the real execution order helps you write better queries and avoid mistakes.

Here's how it works:

1. FROM / JOINs – Where the data comes from

Before anything else, the database needs to know which tables are involved. That means FROM and any JOIN operations run first. This step pulls in the raw data that the rest of the query will work with.

For example:

 SELECT name, order_total
FROM customers
JOIN orders ON customers.id = orders.customer_id;

In this case, the database starts by joining the customers and orders tables. If you're using multiple joins, they're usually processed in the order they appear, though some databases might optimize that differently.

Tip:

• Make sure your joins are efficient here, especially if working with large tables.
• Use aliases early ( AS ) — it makes later steps easier to read.

2. WHERE – Filtering rows

Once the data is pulled in from the tables, the WHERE clause filters out rows that don't match the conditions. Only the matching rows go on to the next steps.

Example:

 WHERE order_total > 100

This would keep only orders over $100 from the earlier joined data.

Common mistake:
Putting filtering logic in HAVING instead of WHERE when it could have been done earlier. That makes the query less efficient.

3. GROUP BY – Organizing data into groups

If you're aggregating data (like using SUM , COUNT , etc.), this is where the database organizes rows into groups based on one or more columns.

Example:

 GROUP BY customers.id

Now all rows are grouped by each customer's ID, so aggregate functions can calculate per group.

Note:

• You can group by expressions too, not just column names.
• Anything in SELECT that isn't an aggregate must be in GROUP BY .

4. HAVING – Filtering groups

After grouping, you may want to filter which groups stay in the result. That's what HAVING does.

Example:

 HAVING COUNT(orders.id) > 5

This keeps only customers who placed more than five orders.

Important:

• HAVING works like WHERE but for aggregated values.
• It only makes sense after grouping.

5. SELECT – Choosing what to return

Only now does the database look at what columns or expressions you want in the final output. This is when calculated fields like SUM(order_total) get their final shape.

Also, column aliases are defined here:

 SELECT customers.name AS customer_name, SUM(orders.total) AS total_spent

But remember: you can't use customer_name until after this step.

6. ORDER BY – Sorting the results

Finally, the results are sorted based on the columns or expressions you specify.

Example:

 ORDER BY total_spent DESC

This sorts customers by how much they spent, from highest to lowest.

Quick tip:
Using expressions in ORDER BY is allowed, but it can sometimes affect performance if not used carefully.

So the full execution order is roughly:

1. FROM / JOINs
2. WHERE
3. GROUP BY
4. HAVING
5. SELECT
6. ORDER BY

And even though you write a SQL query starting with SELECT , that's actually near the end in terms of what the database does.

That's how it works — not complicated, but easy to mix up if you're thinking line-by-line.

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