Working with collation

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About collation

Collation determines how strings are sorted and compared in collation-supported operations. If you would like to customize collation for a collation-supported operation, you must assign a collation specification to at least one string in the operation. Some operations can't use collation, but can pass collation through them.

Operations affected by collation

When an operation is affected by collation, this means that the operation takes into consideration collation during the operation. These query operations are affected by collation when sorting and comparing strings:

Operations Notes
Collation-supported comparison operations
Join operations
ORDER BY In the WINDOW clause, ORDER BY is not supported with collation.
GROUP BY
WINDOW for window functions In the WINDOW clause, ORDER BY and PARTITION BY is not supported with collation.
Collation-supported scalar functions
Collation-supported aggregate functions
Set operations

Operations that propagate collation

Collation can pass through some query operations to other parts of a query. When collation passes through an operation in a query, this is known as propagation. During propagation:

  • If an input contains no collation specification or an empty collation specification and another input contains an explicitly defined collation, the explicitly defined collation is used for all of the inputs.
  • All inputs with a non-empty explicitly defined collation specification must have the same type of collation specification, otherwise an error is thrown.

BigQuery has several functions, operators, and expressions that can propagate collation.

In the following example, the 'und:ci' collation specification is propagated from the character column to the ORDER BY operation.

-- With collation
SELECT *
FROM UNNEST([
  COLLATE('B', 'und:ci'),
  'b',
  'a'
]) AS character
ORDER BY character

+-----------+
| character |
+-----------+
| a         |
| B         |
| b         |
+-----------+
-- Without collation
SELECT *
FROM UNNEST([
  'B',
  'b',
  'a'
]) AS character
ORDER BY character

+-----------+
| character |
+-----------+
| B         |
| a         |
| b         |
+-----------+

Functions

These functions let collation propagate through them:

Function Notes
AEAD.DECRYPT_STRING
ANY_VALUE
ARRAY_AGG Collation on input arguments are propagated as collation on the array element.
ARRAY_TO_STRING Collation on array elements are propagated to output.
COLLATE
CONCAT
FORMAT Collation from format_string to the returned string is propagated.
FORMAT_DATE Collation from format_string to the returned string is propagated.
FORMAT_DATETIME Collation from format_string to the returned string is propagated.
FORMAT_TIME Collation from format_string to the returned string is propagated.
FORMAT_TIMESTAMP Collation from format_string to the returned string is propagated.
GREATEST
LAG
LEAD
LEAST
LEFT
LOWER
LPAD
MAX
MIN
NET.HOST
NET.PUBLIC_SUFFIX
NET.REG_DOMAIN
NORMALIZE
NORMALIZE_AND_CASEFOLD
REPEAT
REPLACE
REVERSE
RIGHT
RPAD
SOUNDEX
SPLIT Collation on input arguments are propagated as collation on the array element.
STRING_AGG
SUBSTR
UPPER

Operators

These operators let collation propagate through them:

Operator Notes
|| concatenation operator
Array subscript operator Propagated to output.
Set operators Collation of an output column is decided by the collations of input columns at the same position.
STRUCT field access operator When getting a STRUCT, collation on the STRUCT field is propagated as the output collation.
UNNEST Collation on the input array element is propagated to output.

Expressions

These expressions let collation propagate through them:

Expression Notes
ARRAY When you construct an ARRAY, collation on input arguments is propagated on the elements in the ARRAY.
CASE
CASE expr
COALESCE
IF
IFNULL
NULLIF
STRUCT When you construct a STRUCT, collation on input arguments is propagated on the fields in the STRUCT.

Where you can assign a collation specification

A collation specification can be assigned to these collation-supported types:

  • A STRING
  • A STRING field in a STRUCT
  • A STRING element in an ARRAY

In addition:

  • You can assign a default collation specification to a schema when you create or alter it. This assigns a default collation specification to all future tables that are added to the schema if the tables do not have their own default collation specifications.
  • You can assign a default collation specification to a table when you create or alter it. This assigns a collation specification to all future collation-supported columns that are added to the table if the columns do not have collation specifications. This overrides a default collation specification on a schema.
  • You can assign a collation specification to a collation-supported type in a column. A column that contains a collation-supported type in its column schema is a collation-supported column. This overrides a default collation specification on a table.
  • You can assign a collation specification to a collation-supported query operation.
  • You can assign a collation specification to a collation-supported expression with the COLLATE function. This overrides any collation specifications set previously.

In summary:

You can define a default collation specification for a schema. For example:

CREATE SCHEMA (...)
DEFAULT COLLATE 'und:ci'

You can define a default collation specification for a table. For example:

CREATE TABLE (...)
DEFAULT COLLATE 'und:ci'

You can define a collation specification for a collation-supported column. For example:

CREATE TABLE (
  case_insensitive_column STRING COLLATE 'und:ci'
)

You can specify a collation specification for a collation-supported expression with the COLLATE function. For example:

SELECT COLLATE('a', 'und:ci') AS character

DDL statements

Location Support Notes
Schema CREATE SCHEMA Create a schema and optionally add a default collation specification to the schema.
Schema ALTER SCHEMA Updates the default collation specification for a schema.
Table CREATE TABLE Create a table and optionally add a default collation specification to a table or a collation specification to a collation-supported type in a column.

You cannot have collation on a column used with CLUSTERING.

You can use collated-supported functions with AS SELECT when creating a table.
Table ALTER TABLE Update the default collation specification for collation-supported type in a table.
Column ADD COLUMN Add a collation specification to a collation-supported type in a new column in an existing table.

Data types

Type Support Notes
Data type STRING You can apply a collation specification directly to this data type.
Data type STRUCT You can apply a collation specification to a STRING field in a STRUCT. A STRUCT can have STRING fields with different collation specifications.
Data type ARRAY You can apply a collation specification to a STRING element in an ARRAY. An ARRAY can have STRING elements with different collation specifications.

Use the COLLATE function to apply a collation specification to collation-supported expressions.

Functions, operators, and conditional expressions

Functions

Type Support Notes
Scalar COLLATE
Scalar ENDS_WITH
Scalar GREATEST
Scalar INSTR
Scalar LEAST
Scalar REPLACE
Scalar SPLIT
Scalar STARTS_WITH
Scalar STRPOS
Aggregate COUNT This operator is only affected by collation when the input includes the DISTINCT argument.
Aggregate MAX
Aggregate MIN

Operators

Support Notes
<
<=
>
>=
=
!=
[NOT] BETWEEN
[NOT] IN This operator generally supports collation, however, [NOT] IN UNNEST is not supported. If used with a list, requires at least one item in the list.
[NOT] IN

Conditional expressions

Support
CASE
CASE expr
NULLIF

The preceding collation-supported operations (functions, operators, and conditional expressions) can include input with explicitly defined collation specifications for collation-supported types. In a collation-supported operation:

  • All inputs with a non-empty, explicitly defined collation specification must be the same, otherwise an error is thrown.
  • If an input does not contain an explicitly defined collation and another input contains an explicitly defined collation, the explicitly defined collation is used for both.

For example:

-- Assume there is a table with this column declaration:
CREATE TABLE table_a
(
    col_a STRING COLLATE 'und:ci',
    col_b STRING COLLATE '',
    col_c STRING,
    col_d STRING COLLATE 'und:ci'
);

-- This runs. Column 'b' has a collation specification and the
-- column 'c' does not.
SELECT STARTS_WITH(col_b_expression, col_c_expression)
FROM table_a;

-- This runs. Column 'a' and 'd' have the same collation specification.
SELECT STARTS_WITH(col_a_expression, col_d_expression)
FROM table_a;

-- This runs. Even though column 'a' and 'b' have different
-- collation specifications, column 'b' is considered the default collation
-- because it's assigned to an empty collation specification.
SELECT STARTS_WITH(col_a_expression, col_b_expression)
FROM table_a;

-- This works. Even though column 'a' and 'b' have different
-- collation specifications, column 'b' is updated to use the same
-- collation specification as column 'a'.
SELECT STARTS_WITH(col_a_expression, COLLATE(col_b_expression, 'und:ci'))
FROM table_a;

-- This runs. Column 'c' does not have a collation specification, so it uses the
-- collation specification of column 'd'.
SELECT STARTS_WITH(col_c_expression, col_d_expression)
FROM table_a;

Collation specification details

A collation specification determines how strings are sorted and compared in collation-supported operations. You can define the Unicode collation specification, und:ci, for collation-supported types.

If a collation specification is not defined, the default collation specification is used. To learn more, see the next section.

Default collation specification

When a collation specification is not assigned or is empty, 'binary' collation is used. Binary collation indicates that the operation should return data in Unicode code point order. You cannot set binary collation explicitly.

Unicode collation specification

collation_specification:
  'language_tag:collation_attribute'

A unicode collation specification indicates that the operation should use the Unicode Collation Algorithm to sort and compare strings. The collation specification can be a STRING literal or a query parameter.

The language tag

The language tag determines how strings are generally sorted and compared. Allowed values for language_tag are:

  • und: A locale string representing the undetermined locale. und is a special language tag defined in the IANA language subtag registry and used to indicate an undetermined locale. This is also known as the root locale and can be considered the default Unicode collation. It defines a reasonable, locale agnostic collation.

The collation attribute

In addition to the language tag, the unicode collation specification must have a collation_attribute, which enables additional rules for sorting and comparing strings. Allowed values are:

  • ci: Collation is case-insensitive.

Collation specification example

This is what the ci collation attribute looks like when used with the und language tag in the COLLATE function:

COLLATE('orange1', 'und:ci')

Caveats

  • Differing strings can be considered equal. For instance, (LATIN CAPITAL LETTER SHARP S) is considered equal to 'SS' on primary level thus 'ẞ1' < 'SS2'. This is similar to how case insensitivity works.
  • There are a wide range of unicode code points (punctuation, symbols, etc), that are treated as if they are not there. So strings with and without them are sorted identically. For example, the format control code point U+2060 is ignored when the following strings are sorted:

    SELECT *
    FROM UNNEST([
      COLLATE('oran\u2060ge1', 'und:ci'),
      COLLATE('\u2060orange2', 'und:ci'),
      COLLATE('orange3', 'und:ci')
    ]) AS fruit
    ORDER BY fruit
    
    +---------+
    | fruit   |
    +---------+
    | orange1 |
    | orange2 |
    | orange3 |
    +---------+
    
  • Ordering may change. The Unicode specification of the und collation can change occasionally, which can affect sorting order.

Limitations

Limitations for supported features are captured in the previous sections, but here are a few general limitations to keep in mind:

  • und:ci and empty collation are supported, but not other collation specifications.
  • Operations and functions that do not support collation produce an error if they encounter collated values.
  • You cannot set non-empty collation on a clustering field, using an API call or DDL query.

    CREATE TABLE mydataset.mytable
    (
      word STRING COLLATE 'und:ci',
      number INT64
    )
    CLUSTER BY word;
    
    -- User error:
    -- “CLUSTER BY STRING column word with
    -- collation und:ci is not supported"
    
  • A materialized view with collated output columns is not supported.

    CREATE MATERIALIZED VIEW bigquery.public.data.samples.shakespeare
    AS SELECT COLLATE('abc', 'und:ci') x;
    
    -- User error:
    -- “Creating MATERIALIZED VIEW with collation
    -- on the output column is not supported”
    
  • A view with collated output columns is not supported.

    CREATE VIEW bigquery.public.data.samples.shakespeare
    AS SELECT COLLATE('abc', 'und:ci') x;
    
    -- User error:
    -- "Creating VIEW with collation on the output column
    -- is not supported"
    
  • After a view or materialized view is created and a column collation of the base table is altered to make the column collated, the view or materialized view is invalidated and cannot be queried.

    CREATE TABLE dataset.base (
      col_no_collation STRING,
      col INT64
    )
    AS SELECT 'a', 1
    UNION ALL SELECT 'b', 2
    UNION ALL SELECT 'A', 3
    UNION ALL SELECT 'B', 4;
    
    CREATE VIEW/MATERIALIZED VIEW dataset.view AS SELECT * FROM dataset.base;
    
    -- Change the collation of string column of the base table so
    -- that the string column of view/materialized view is collated.
    ALTER TABLE dataset.base ALTER COLUMN col_no_collation SET DATA TYPE STRING COLLATE 'und:ci';
    
    -- Should return an error since the column of view/materialized
    -- contains collation.
    SELECT * FROM dataset.view ORDER BY col;
    
    -- User error:
    -- "Collation on the output column for view dataset.view is
    -- not supported"
    
  • You cannot create a materialized view with collated sort keys in an aggregate function.

    CREATE TABLE dataset.base (
      col_no_collation STRING,
      col INT64
    )
    AS SELECT 'a', 1
    UNION ALL SELECT 'b', 2
    UNION ALL SELECT 'A', 3
    UNION ALL SELECT 'B', 4;
    
    CREATE VIEW/MATERIALIZED VIEW dataset.view AS SELECT * FROM dataset.base;
    
    -- Change the collation of string column of the base table so
    -- that the string column of view/materialized view is
    -- collated.
    ALTER TABLE dataset.base ALTER COLUMN col_no_collation SET DATA TYPE STRING COLLATE 'und:ci';
    
    -- Should return an error since the column of view/materialized
    -- contains collation.
    SELECT * FROM dataset.view ORDER BY col;
    
    -- User error:
    -- "Collation on the output column for view dataset.view is
    -- not supported"
    
  • A table function with collated output columns is not supported.

    CREATE TABLE FUNCTION bigquery.public.data.samples.shakespeare()
    AS SELECT COLLATE('abc', 'und:ci') x;
    
    -- User error:
    -- "Creating TABLE FUNCTION with collation on the output
    -- column is not supported"
    
  • User defined functions (UDFs) cannot take collated arguments.

    CREATE FUNCTION tmp_dataset.f(x STRING) AS (x);
    
    SELECT tmp_dataset.f(col_ci)
    FROM shared_dataset.table_collation_simple
    
    -- User error:
    -- “Collation is not allowed on argument x ("und:ci").
    -- Use COLLATE(arg, '') to remove collation at [1:8]”
    
  • If a SELECT query with a user-specified destination table produces output columns with collation, an error is produced.

    -- The following query returns an error since the resulting schema of
    -- the query has a column string_ci with collation ‘und:ci’.
    SELECT string_ci FROM collated_table
    
  • BigQuery BI engine does not support tables with collation or the COLLATE function.