The error appears when an OUTER JOIN’s ON clause references tables in a cross-dependent way that violates MySQL’s join rules.
MySQL Error 1120: ER_WRONG_OUTER_JOIN signals a cross dependency inside the OUTER JOIN’s ON clause. Re-write the join so each table in the ON clause only references columns from tables that appear before it in the query; removing the circular reference resolves the error.
Cross dependency found in OUTER JOIN; examine your ON
MySQL raises Error 1120 with the message “Cross dependency found in OUTER JOIN; examine your ON conditions” when the ON clause of a LEFT or RIGHT OUTER JOIN contains circular references. The engine cannot determine the correct evaluation order and aborts parsing.<\/p>
The problem is purely syntactic.
Data files and server connections stay intact, but the statement will never run until you restructure the JOIN so that each ON condition references only tables already introduced on the left side of the join chain.<\/p>
Cross dependencies occur when columns from the right-hand table are used to filter or compare columns from the left-hand table in the same ON clause.
MySQL demands a one-directional dependency tree during OUTER JOIN parsing.<\/p>
The error also surfaces when developers mistakenly move predicate logic that belongs in the WHERE clause into the ON clause, or when they reorder tables without updating the join predicates accordingly.<\/p>
Identify predicates in the ON clause that reference tables introduced later in the join list.
Move those predicates to a subsequent JOIN or to the WHERE clause, or rewrite the query using derived tables. Ensure each ON clause only uses tables defined earlier in the statement.<\/p>
After the rewrite, validate the plan with EXPLAIN to confirm MySQL can build the join order without circular references.<\/p>
Reports that join a fact table to two dimension tables often add predicates that link the dimensions together.
Moving those predicates into a separate INNER JOIN or a subquery clears the error.<\/p>
ETL pipelines that right-join staging data to target tables sometimes compare staging columns inside the ON clause. Switching to LEFT JOIN or filtering in WHERE usually eliminates the cross dependency.<\/p>
Always write join predicates so that each new table depends only on tables to its left.
Keep filtering conditions that involve multiple tables in the WHERE clause unless the logic requires them inside ON.<\/p>
Use table aliases, consistent join order, and Galaxy’s SQL linting to spot circular references early. Automated unit tests that run EXPLAIN on critical queries further reduce risk.<\/p>
Syntax errors like ER_PARSE_ERROR or ER_NONUNIQ_TABLE may appear during complex rewrites.
Ensuring unique aliases and correct parentheses often solves these auxiliary issues.<\/p>
Logical mistakes such as ER_BAD_FIELD_ERROR arise when columns are moved between clauses. Verifying column existence with DESCRIBE before executing the adjusted query prevents them.<\/p>.
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