1. on condition prior to the execution where conditions . on the proviso that when executed generate a temporary table, where the data is filtered in the temporary table generation.
2. put aside where conditions, see On condition . Regardless on whether the conditions become, the left table will select all rows of data out (regardless of whether the right conditions for the establishment of table), select the right table set up on the corresponding data set is not set up to null. There may be many.
When two tables are linked to the left join, the conditions followed on: Whether on condition is satisfied, the return value will be left table;
When two tables are linked to the left join, when the conditions in the back where: only records that satisfy the condition will return.