Collations

Collations provide rules for how text should be sorted or compared in the execution engine. Collations are useful for localization, as the rules for how text should be ordered are different for different languages or for different countries. These orderings are often incompatible with one another. For example, in English the letter "y" comes between "x" and "z". However, in Lithuanian the letter "y" comes between the "i" and "j". For that reason, different collations are supported. The user must choose which collation they want to use when performing sorting and comparison operations.

By default, the BINARY collation is used. That means that strings are ordered and compared based only on their binary contents. This makes sense for standard ASCII characters (i.e. the letters A-Z and numbers 0-9), but generally does not make much sense for special unicode characters. It is, however, by far the fastest method of performing ordering and comparisons. Hence it is recommended to stick with the BINARY collation unless required otherwise.

Using Collations

In the stand-alone installation of DuckDB three collations are included: NOCASE, NOACCENT and NFC. The NOCASE collation compares characters as equal regardless of their casing. The NOACCENT collation compares characters as equal regardless of their accents. The NFC collation performs NFC-normalized comparisons, see here for more information.

SELECT 'hello'='hElLO';
-- false
SELECT 'hello' COLLATE NOCASE='hElLO';
-- true

SELECT 'hello' = 'hëllo';
-- false
SELECT 'hello' COLLATE NOACCENT = 'hëllo';
-- true

Collations can be combined by chaining them using the dot operator. Note, however, that not all collations can be combined together. In general, the NOCASE collation can be combined with any other collator, but most other collations cannot be combined.

SELECT 'hello' COLLATE NOCASE='hElLÖ';
-- false
SELECT 'hello' COLLATE NOACCENT='hElLÖ';
-- false
SELECT 'hello' COLLATE NOCASE.NOACCENT='hElLÖ';
-- true

Default Collations

The collations we have seen so far have all been specified per expression. It is also possible to specify a default collator, either on the global database level or on a base table column. The PRAGMA default_collation can be used to specify the global default collator. This is the collator that will be used if no other one is specified.

PRAGMA default_collation=NOCASE;

SELECT 'hello'='HeLlo';
-- true

Collations can also be specified per-column when creating a table. When that column is then used in a comparison, the per-column collation is used to perform that comparison.

CREATE TABLE names(name VARCHAR COLLATE NOACCENT);
INSERT INTO names VALUES ('hännes');
SELECT name FROM names WHERE name='hannes';
-- hännes

Be careful here, however, as different collations cannot be combined. This can be problematic when you want to compare columns that have a different collation specified.

SELECT name FROM names WHERE name='hannes' COLLATE NOCASE;
-- ERROR: Cannot combine types with different collation!

CREATE TABLE other_names(name VARCHAR COLLATE NOCASE);
INSERT INTO other_names VALUES ('HÄNNES');

SELECT * FROM names, other_names WHERE names.name=other_names.name;
-- ERROR: Cannot combine types with different collation!

-- need to manually overwrite the collation!

SELECT * FROM names, other_names WHERE names.name COLLATE NOACCENT.NOCASE=other_names.name COLLATE NOACCENT.NOCASE;
-- hännes|HÄNNES

ICU Collations

The collations we have seen so far are not region dependent, and do not follow any specific regional rules. If you wish to follow the rules of a specific region or language, you will need to use one of the ICU collations. For that, you need to include the ICU extension. This can be found in the extension/icu folder in the project. Using the C++ API, the extension can be loaded as follows:

DuckDB db;
db.LoadExtension<ICUExtension>();

Loading this extension will add a number of language and region specific collations to your database. These can be queried using PRAGMA collations command, or by querying the pragma_collations function.

PRAGMA collations;
SELECT * FROM pragma_collations();
-- [af, am, ar, as, az, be, bg, bn, bo, bs, bs, ca, ceb, chr, cs, cy, da, de, de_AT, dsb, dz, ee, el, en, en_US, en_US, eo, es, et, fa, fa_AF, fi, fil, fo, fr, fr_CA, ga, gl, gu, ha, haw, he, he_IL, hi, hr, hsb, hu, hy, id, id_ID, ig, is, it, ja, ka, kk, kl, km, kn, ko, kok, ku, ky, lb, lkt, ln, lo, lt, lv, mk, ml, mn, mr, ms, mt, my, nb, nb_NO, ne, nl, nn, om, or, pa, pa, pa_IN, pl, ps, pt, ro, ru, se, si, sk, sl, smn, sq, sr, sr, sr_BA, sr_ME, sr_RS, sr, sr_BA, sr_RS, sv, sw, ta, te, th, tk, to, tr, ug, uk, ur, uz, vi, wae, wo, xh, yi, yo, zh, zh, zh_CN, zh_SG, zh, zh_HK, zh_MO, zh_TW, zu]

These collations can then be used as the other collations would be used before. They can also be combined with the NOCASE collation. For example, to use the German collation rules you could use the following code snippet:

CREATE TABLE strings(s VARCHAR COLLATE DE);
INSERT INTO strings VALUES ('Gabel'), ('Göbel'), ('Goethe'), ('Goldmann'), ('Göthe'), ('Götz');
SELECT * FROM strings ORDER BY s;
-- "Gabel", "Göbel", "Goethe", "Goldmann", "Göthe", "Götz"