Format and beautify SQL queries online

No. Only whitespace and line breaks change. The one thing to check is a string literal containing meaningful whitespace, which is left untouched.

Because each engine adds syntax the others reject. PostgreSQL casts with a double colon, MySQL quotes identifiers with backticks, and BigQuery allows a trailing comma in a select list. Choosing the wrong dialect can produce a parse error on valid SQL.

Usually a dialect specific construct with the wrong dialect selected, or a templating placeholder that is not valid SQL. Try the matching dialect first, and replace template placeholders with real values to check.

Only enough to parse it for formatting. It cannot tell you whether a table or column exists, and it does not run anything.

No. The formatter runs in the browser, so a query containing table names or literal values from production stays local.