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 CREATE TABLE query.

Engine Parameters

  • join_strictnessJOIN strictness.
  • join_typeJOIN type.
  • k1[, k2, ...] – Key columns from the USING clause that the JOIN operation is made with.

Set the parameters join_strictness and join_type without quotes, for example, Join(ANY, LEFT, col1). They must match the JOIN operation that the table will be used for. If 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 │
└────┴─────┴─────────────────┘

Retrieving the 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 to add data to the table. For the ANY strictness, data for duplicated keys are ignored. For the ALL strictness, all rows are kept.

You cannot perform the SELECT query directly from the table. Use one of the following ways:

  • Place the table at the right side in a JOIN clause.
  • Call the joinGet function, which allows to extract data from the table as from a dictionary.

Limitations and Settings

When creating a table, the following settings are applied:

The table can't be used in GLOBAL JOIN operations.

Data Storage

Data for the Join tables is always located in RAM. When inserting rows into the table, ClickHouse writes the data blocks to the directory on disk to be able to restore them on server restart.

At the abnormal server restart, the block of data on the disk might be lost or damaged. In this case, you may need to manually delete the file with damaged data.