What do you do if your database queries are not returning the expected results?

Before tackling complex troubleshooting, begin with the fundamentals: check the syntax of your query. Even seasoned database developers can miss a misplaced comma or an incorrect operator. SQL is very strict about syntax, and even minor mistakes can cause errors or yield unexpected results. Utilize your DBMS’s syntax checker or execute simplified versions of the query to pinpoint any syntax problems.

For a start verify your syntax accuracy. Start by examining the error message, if it is not very informing , focus on what is near it , for data type compatibility , check column types and conversion functions, ensure you are using the correct conversions , utilizing correct character sets and formats. Check for missing characters in your query , maybe you need to swap a "sleect" with "select". If the syntax errors come from a migration attempt , check the documentation of the new system. Ensure your joins are on point, verify the naming of the columns, missing aliases in subqueries, missing commas in functions, missing quote marks, missing semicolons, unclosed parentheses, non English characters... and more.

The most difficult to find issues , are ones revolving around data, an enormous number of things can go wrong but with adequate commitment and experience , you will find the solution. Check for: field size violations, invalid data type insertion attempts, improper date format in comparisons for example DD/MM/YYYY >= MM/DD/YYYY, wrong order of end columns in insert into select clauses, violation of format conventions, subtle automatic type castings in "case when" structures based on the first line of output, improper encoding between interconneted files, inaccurate character counting leading to usage of the wrong character, improper ascii comparisons, non existence of necessary special characters, absence of defense, nulls and many more.

After verifying that your syntax and data are correct, it's essential to examine the logic of your query. Complex queries, particularly those involving multiple joins or subqueries, can contain logical errors that aren't immediately obvious. To troubleshoot, break the query down into smaller components and test each part separately. This approach can help you identify where the logical breakdown occurs. Grasping the logic flow is key to ensuring that the query functions as expected.

A key technique in programming, is to avoid doing complex calculations in your head only. When facing a query with an erroneous outcome, one should break it down in parts ,and verify each block seperately. start from the first and most restrictive joins, verify correct binding of fields , date intervals, and various comparisons. Then start adding the rest of the tables in the join, verifying the outcome. In inner joins , pay attention to nullable fields. The outer joins should come last, there you must pay attention to null values and comparisons to them because the DB ends up with an "UNKNOWN" outcome , instead of "TRUE" or "FALSE", so pay attention to this ternary logic. Of course examine the where , group and aggregations too.