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Merge branch 'master' of github.com:yandex/ClickHouse

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BayoNet 2019-08-21 13:47:14 +03:00
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35
docs/en/query_language/select.md
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@ -547,18 +547,35 @@ ClickHouse doesn't directly support syntax with commas, so we don't recommend us
Tables for `ASOF JOIN` must have an ordered sequence column. This column cannot be alone in a table, and should be one of the data types: `UInt32`, `UInt64`, `Float32`, `Float64`, `Date`, and `DateTime`. Tables for `ASOF JOIN` must have an ordered sequence column. This column cannot be alone in a table, and should be one of the data types: `UInt32`, `UInt64`, `Float32`, `Float64`, `Date`, and `DateTime`.
Use the following syntax for `ASOF JOIN`: You can use the following types of syntax:
``` - `ASOF JOIN ... ON`
SELECT expression_list FROM table_1 ASOF JOIN table_2 USING(equi_column1, ... equi_columnN, asof_column)
```
`ASOF JOIN` uses `equi_columnX` for joining on equality (`user_id` in our example) and `asof_column` for joining on the closest match. ```sql
SELECT expressions_list
FROM table_1
ASOF LEFT JOIN table_2
ON equi_cond AND closest_match_cond
```
You can use any number of equality conditions and exactly one closest match condition. For example, `SELECT count() FROM A ASOF LEFT JOIN B ON A.a == B.b AND B.t <= A.t`. There is just `table_2.some_col <= table_1.some_col` and `table_1.some_col >= table2.some_col` types of conditions are available. You cannot apply other conditions like `>` or `!=`.
- `ASOF JOIN ... USING`
```sql
SELECT expressions_list
FROM table_1
ASOF JOIN table_2
USING (equi_column1, ... equi_columnN, asof_column)
```
`ASOF JOIN` uses `equi_columnX` for joining on equality and `asof_column` for joining on the closest match with the `table_1.asof_column >= table2.asof_column` condition. The `asof_column` column must be the last in the `USING` clause.
For example, consider the following tables: For example, consider the following tables:
``` ```
table_1 table_2 table_1 table_2
event | ev_time | user_id event | ev_time | user_id event | ev_time | user_id event | ev_time | user_id
----------|---------|---------- ----------|---------|---------- ----------|---------|---------- ----------|---------|----------
... ... ... ...
@ -568,13 +585,11 @@ event_1_2 | 13:00 | 42 event_2_3 | 13:00 | 42
... ... ... ...
``` ```
`ASOF JOIN` takes the timestamp of a user event from `table_1` and finds an event in `table_2` where the timestamp is closest (equal or less) to the timestamp of the event from `table_1`. Herewith the `user_id` column is used for joining on equality and the `ev_time` column is used for joining on the closest match. `ASOF JOIN` can take the timestamp of a user event from `table_1` and find an event in `table_2` where the timestamp is closest (equal or less) to the timestamp of the event from `table_1`. Herewith the `user_id` column can be used for joining on equality and the `ev_time` column can be used for joining on the closest match. In our example, `event_1_1` can be joined with `event_2_1`, `event_1_2` can be joined with `event_2_3`, but `event_2_2` cannot be joined.
In our example, `event_1_1` can be joined with `event_2_1`, `event_1_2` can be joined with `event_2_3`, but `event_2_2` cannot be joined.
Implementation details:
- `asof_column` should be last in the `USING` clause. !!! note "Note"
- `ASOF` join is not supported in the [Join](../operations/table_engines/join.md) table engine. `ASOF` join is **not** supported in the [Join](../operations/table_engines/join.md) table engine.
To set the default strictness value, use the session configuration parameter [join_default_strictness](../operations/settings/settings.md#settings-join_default_strictness). To set the default strictness value, use the session configuration parameter [join_default_strictness](../operations/settings/settings.md#settings-join_default_strictness).