Why Do Multiple SQL INNER JOINs Require Parentheses?

SQL multi-table connection syntax error

When a SQL statement contains multiple INNER JOIN operations, all JOIN clauses except the first JOIN clause must be enclosed in parentheses to avoid syntax errors.

Problem description:

A SQL statement that previously had only one INNER JOIN throws a syntax error after adding a second table join. The sentence is as follows:

<code class="language-sql">adsFormView.SelectCommand = "SELECT * FROM [tableCourse] 
INNER JOIN [tableGrade] ON [tableCourse].[grading] = [tableGrade].[id] 
INNER JOIN [tableCourseType] ON [tableCourse].[course_type] = [tableCourseType].[id] 
WHERE [prefix]='" & myPrefix & "' AND [course_number]='" & myCourseNum & "'"</code>

Solution:

To resolve this error, additional JOIN clauses must be enclosed in parentheses. The corrected sentence is as follows:

<code class="language-sql">adsFormView.SelectCommand = "SELECT * FROM [tableCourse] 
INNER JOIN [tableGrade] ON [tableCourse].[grading] = [tableGrade].[id] 
INNER JOIN (
    [tableCourseType] ON [tableCourse].[course_type] = [tableCourseType].[id]
)
WHERE [prefix]='" & myPrefix & "' AND [course_number]='" & myCourseNum & "'"</code>

Explanation:

For multi-table joins, each additional JOIN operation must be enclosed in parentheses. This ensures correct operator precedence and prevents syntax errors. The innermost JOIN is executed first, then the next outer JOIN, and so on until the original FROM table is reached.

By adhering to this syntax, multi-table joins in SQL statements can be executed correctly.

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