# General WHERE Clause Considerations

- All tables specified in the From Clause must participate in a join statement with another table. Table left not joined, would cause a Cartesian join to be applied for this table and any other table on the list, resulting in an incorrect result list let alone very poor performance.
- Note that there is no such thing as "conditional" join where the only join statement for a table is under a condition. In cases where the condition is not met and thus the join is not performed, one would end up with the same problem described previously.
- The final result set is built up by taking the full population of the tables involved and applying the restricting criteria to it one after another where the intermediate result population of one step is the input for the next step. Therefore, it is recommended to specify the most restrictive criteria first so that at the end of one step, lesser records are processed in the next step.
- This is of course a very schematic and simplified way to describe the internal process. This is not necessarily how the database is processing the statements. However, setting up the criteria as described would direct the database to use the right path.

Parent topic: WHERE Clause