Postgres LEFT JOIN 与 WHERE 条件

Question

I need to left join two tables with a where condition:

Table time_table

id     rid        start_date                end_date
1       2     2017-07-01 00:00:00     2018-11-01 00:00:00
2       5     2017-01-01 00:00:00     2017-06-01 00:00:00
3       2     2018-07-01 00:00:00     2020-11-01 00:00:00

Table record_table

id      name                 date
1      record1       2017-10-01 00:00:00
2      record2       2017-02-01 00:00:00
3      record3       2017-10-01 00:00:00

I need to get all those records which are present under given date range. In the above example, I need those records that lie under range for rid = 2 only. Hence the output for the above query needs to be:

1      record1       2017-10-01 00:00:00    
3      record3       2017-10-01 00:00:00

Answer 1

left join two tables with a where condition

It's typically wrong to use a LEFT [OUTER] JOIN and then filter with a WHERE condition, thereby voiding the special feature of a LEFT JOIN to include all rows from the left table unconditionally . Detailed explanation:

• Explain JOIN vs. LEFT JOIN and WHERE condition performance suggestion in more detail

Put conditions supposed to filter all rows into the WHERE clause ( rid = 2 ), but make conditions to left-join rows from record_table out to be actual join conditions:

SELECT t.start_date, t.end_date  -- adding those
     , r.id, r.name, r.date 
FROM   time_table t
LEFT   JOIN record_table r ON r.date >= t.start_date
                          AND r.date <  t.end_date
WHERE  t.rid = 2;

As commented , it makes sense to include columns from time_table in the result, but that's my optional addition.

You also need to be clear about lower and upper bounds . The general convention is to include the lower and exclude the upper bound in time ( timestamp ) ranges. Hence my use of >= and < above.

Related:

• SQL query on a time series to calculate the average
• Selecting an average of records grouped by 5 minute periods

Performance should be no problem at all with the right indexes. You need an index (or PK) on time_table(rid) and another on record_table(date) .

Answer 2

I'm not exactly sure if this is what you want, but if you are saying you want the dates where the record_table date is between the dates in the time_table, then this would do the job:

select
  rt.id, rt.name, rt.date
from
  time_table tt
  join record_table rt on
    rt.date between tt.start_date and tt.end_date
where
  tt.rid = 2

That said, this will be horribly inefficient for large datasets. If your data is relatively small (< 10k records in each table, post-filters), then it probably won't matter much, but if you would need to scale this concept, it would warrant knowing more about your data -- for example, do the dates, always round to the first of each month?

Again, from your example, I wasn't sure if this is what you meant by "get all those records which are present under given date range."

Answer 3

SELECT time_tbl.name,record_tbl.date 
FROM dbo.time_table AS time_tbl
     INNER JOIN record_table AS record_tbl
           ON time_tbl.id=record_tbl.id
WHERE(time_tbl.rid=2)