为何SQL的JOIN USING子句需使用括号？附实例解析

Great question—let’s break down why those parentheses are non-negotiable, and walk through how the database parses this syntax.

Core Syntax Rule from SQL Standards #

First, the USING clause is defined in SQL standards to require parentheses around the column name(s) it references. This isn’t an arbitrary quirk—it’s designed to handle scenarios where you might join on multiple columns at once. For example:

JOIN TABLE_A USING (col1, col2, col3)

The parentheses act as a clear delimiter, telling the database “these are the exact columns we’re using to link the tables”. Even when you only have one column (like Publisher_Code), the syntax still requires this wrapper—think of it as sticking to a consistent rule regardless of how many columns you’re joining on.

How the Database Parses This Statement #

Let’s walk through what happens when the database reads your example query:

1. It starts processing the JOIN PUBLISHER part, then hits the USING keyword.
2. At this point, the database’s parser expects to see an opening parenthesis ( to signal the start of the column list.
3. If you omit the parentheses (writing USING Publisher_Code instead), the parser gets confused. It will try to interpret Publisher_Code as part of an incomplete syntax, then hit the next JOIN WROTE and realize this doesn’t fit any valid SQL structure. This mismatch triggers a syntax error immediately.

To put it simply: without parentheses, the database can’t tell where the USING clause ends and the next part of your query begins. The parentheses act as a “container” that clearly bounds the columns for the join.

Your Example Query in Context #

Here’s your full query for reference:

SELECT Book_Title as 'Title', Publisher_Name 
FROM BOOK 
JOIN PUBLISHER USING (Publisher_Code) 
JOIN WROTE USING (Book_Code) 
JOIN AUTHOR USING (Author_Num) 
GROUP BY Book_Title, Publisher_Name 
HAVING COUNT(DISTINCT CONCAT(LastName,FirstName)) = 2

If you removed any of the parentheses (say, from USING (Publisher_Code)), the database would throw an error because it can’t resolve the invalid syntax. The parentheses ensure each USING clause is cleanly separated from the subsequent joins and clauses.

Quick Comparison to ON Clause #

You might wonder why ON doesn’t need parentheses (most of the time). That’s because ON expects a logical condition (like BOOK.Publisher_Code = PUBLISHER.Publisher_Code), not a list of columns. The syntax rules for ON and USING are distinct, which is why their requirements differ.

内容的提问来源于stack exchange，提问作者user11621936