4.7 KiB
toc_priority | toc_title |
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40 | Join |
Join
Prepared data structure for using in JOIN operations.
Creating a Table
CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
(
name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1] [TTL expr1],
name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2] [TTL expr2],
) ENGINE = Join(join_strictness, join_type, k1[, k2, ...])
See the detailed description of the CREATE TABLE query.
Engine Parameters
join_strictness
– JOIN strictness.join_type
– JOIN type.k1[, k2, ...]
– Key columns from theUSING
clause that theJOIN
operation is made with.
Enter join_strictness
and join_type
parameters without quotes, for example, Join(ANY, LEFT, col1)
. They must match the JOIN
operation that the table will be used for. If the parameters don’t match, ClickHouse doesn’t throw an exception and may return incorrect data.
Table Usage
Example
Creating the left-side table:
CREATE TABLE id_val(`id` UInt32, `val` UInt32) ENGINE = TinyLog
INSERT INTO id_val VALUES (1,11)(2,12)(3,13)
Creating the right-side Join
table:
CREATE TABLE id_val_join(`id` UInt32, `val` UInt8) ENGINE = Join(ANY, LEFT, id)
INSERT INTO id_val_join VALUES (1,21)(1,22)(3,23)
Joining the tables:
SELECT * FROM id_val ANY LEFT JOIN id_val_join USING (id) SETTINGS join_use_nulls = 1
┌─id─┬─val─┬─id_val_join.val─┐
│ 1 │ 11 │ 21 │
│ 2 │ 12 │ ᴺᵁᴸᴸ │
│ 3 │ 13 │ 23 │
└────┴─────┴─────────────────┘
As an alternative, you can retrieve data from the Join
table, specifying the join key value:
SELECT joinGet('id_val_join', 'val', toUInt32(1))
┌─joinGet('id_val_join', 'val', toUInt32(1))─┐
│ 21 │
└────────────────────────────────────────────┘
Selecting and Inserting Data
You can use INSERT
queries to add data to the Join
-engine tables. If the table was created with the ANY
strictness, data for duplicate keys are ignored. With the ALL
strictness, all rows are added.
You cannot perform a SELECT
query directly from the table. Instead, use one of the following methods:
- Place the table to the right side in a
JOIN
clause. - Call the joinGet function, which lets you extract data from the table the same way as from a dictionary.
Limitations and Settings
When creating a table, the following settings are applied:
The Join
-engine tables can’t be used in GLOBAL JOIN
operations.
The Join
-engine allows use join_use_nulls setting in the CREATE TABLE
statement. And SELECT query allows use join_use_nulls
too. If you have different join_use_nulls
settings, you can get an error joining table. It depends on kind of JOIN. When you use joinGet function, you have to use the same join_use_nulls
setting in CRATE TABLE
and SELECT
statements.
Data Storage
Join
table data is always located in the RAM. When inserting rows into a table, ClickHouse writes data blocks to the directory on the disk so that they can be restored when the server restarts.
If the server restarts incorrectly, the data block on the disk might get lost or damaged. In this case, you may need to manually delete the file with damaged data.