Nested(name1 Type1, Name2 Type2, …)

Example:

This example declares the nested data structure, which contains data about conversions (goals reached). Each row in the ‘visits’ table can correspond to zero or any number of conversions.

Only a single nesting level is supported. Columns of nested structures containing arrays are equivalent to multidimensional arrays, so they have limited support (there is no support for storing these columns in tables with the MergeTree engine).

Example:

It is easiest to think of a nested data structure as a set of multiple column arrays of the same length.

The only place where a SELECT query can specify the name of an entire nested data structure instead of individual columns is the ARRAY JOIN clause. For more information, see “ARRAY JOIN clause”. Example:

For an INSERT query, you should pass all the component column arrays of a nested data structure separately (as if they were individual column arrays). During insertion, the system checks that they have the same length.

For a DESCRIBE query, the columns in a nested data structure are listed separately in the same way.

The ALTER query for elements in a nested data structure has limitations.