ClickHouse/docs/en/engines/table-engines/special/join.md

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40	Join

Join Table Engine

Optional prepared data structure for usage in JOIN operations.

!!! note "Note" This is not an article about the JOIN clause itself.

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 the USING clause that the JOIN 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:

• join_use_nulls
• max_rows_in_join
• max_bytes_in_join
• join_overflow_mode
• join_any_take_last_row

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.

Original article