Beautiful SQL: Lateral Unnesting of Array Columns

Sometimes, SQL can just be so beautiful. One of the less mainstream features in SQL is the array type (or nested collections). In fact, it’s so not mainstream that only 2 major databases actually support it: Oracle and PostgreSQL (and HSQLDB and H2 in the Java ecosystem).

In PostgreSQL, you can write:

CREATE TABLE blogs (
  id    SERIAL NOT NULL PRIMARY KEY,
  title text   NOT NULL,
  tags  text[]
)

Or in Oracle:

-- Oracle only knows nominal array types, so we have to declare
-- them in advance
CREATE TYPE tag_t AS VARRAY(100) OF VARCHAR2(100 CHAR);

CREATE TABLE blogs (
  id    NUMBER(18) GENERATED BY DEFAULT AS IDENTITY 
                   NOT NULL PRIMARY KEY,
  title VARCHAR2(100 CHAR) NOT NULL,
  tags  tag_t
)

So, roughly the same thing. Now, let’s insert some data. How about the 3 most recent posts on the jOOQ blog, prior to this one:

In PostgreSQL:

INSERT INTO blogs (title, tags)
VALUES (
  'How to Fetch Oracle 12c Implicit Cursors with JDBC and jOOQ',
  ARRAY[
    'implicit cursor',
    'batch',
    'oracle',
    'jooq',
    'jdbc',
    'resultset'
  ]
), (
  'How to Execute SQL Batches With JDBC and jOOQ',
  ARRAY[
    'batch',
    'batch statement',
    'mysql',
    'jooq',
    'jdbc',
    'sql server',
    'sql'
  ]
), (
  'How to Emulate Partial Indexes in Oracle',
  ARRAY[
    'optimisation',
    'index',
    'partial index',
    'oracle',
    'sql',
    'postgresql',
    't-sql',
    'sql server'
  ]
)

Or in Oracle:

INSERT INTO blogs (title, tags)
VALUES (
  'How to Fetch Oracle 12c Implicit Cursors with JDBC and jOOQ',
  tag_t(
    'implicit cursor',
    'batch',
    'oracle',
    'jooq',
    'jdbc',
    'resultset'
  ));
INSERT INTO blogs (title, tags)
VALUES (
  'How to Execute SQL Batches With JDBC and jOOQ',
  tag_t(
    'batch',
    'batch statement',
    'mysql',
    'jooq',
    'jdbc',
    'sql server',
    'sql'
  ));
INSERT INTO blogs (title, tags)
VALUES (
  'How to Emulate Partial Indexes in Oracle',
  tag_t(
    'optimisation',
    'index',
    'partial index',
    'oracle',
    'sql',
    'postgresql',
    't-sql',
    'sql server'
  ));

Now, the array type by itself is not very useful. When it gets really interesting is when we unnest it again into a table. For instance in PostgreSQL:

SELECT title, tag
FROM blogs, LATERAL unnest(tags) AS tags(tag);

Or in Oracle:

-- Classic style
SELECT title, tags.*
FROM blogs, TABLE(tags) tags;

-- Since Oracle 12c
SELECT title, tags.*
FROM blogs, LATERAL (SELECT * FROM TABLE(tags)) tags;

Note that we’re using the keyword LATERAL in some of the above queries. For those of you who are used to T-SQL syntax, it’s almost the same thing as APPLY. Both LATERAL and APPLY are also very useful with table valued functions (stay tuned for a blog post on those).

The idea behind LATERAL is that the table (derived table, subquery, function call, array unnesting) on the right side of LATERAL can “laterally” access stuff from the left side of LATERAL in order to produce new tables. In the above query, we’re producing a new table of tags for each blog post, and then we cross join the two tables.

Here’s what the above queries result in:

title                                                         tag
-----------------------------------------------------------------------------
How to Fetch Oracle 12c Implicit Cursors with JDBC and jOOQ   implicit cursor
How to Fetch Oracle 12c Implicit Cursors with JDBC and jOOQ   batch
How to Fetch Oracle 12c Implicit Cursors with JDBC and jOOQ   oracle
How to Fetch Oracle 12c Implicit Cursors with JDBC and jOOQ   jooq
How to Fetch Oracle 12c Implicit Cursors with JDBC and jOOQ   jdbc
How to Fetch Oracle 12c Implicit Cursors with JDBC and jOOQ   resultset
How to Execute SQL Batches With JDBC and jOOQ                 batch
How to Execute SQL Batches With JDBC and jOOQ                 batch statement
How to Execute SQL Batches With JDBC and jOOQ                 mysql
How to Execute SQL Batches With JDBC and jOOQ                 jooq
How to Execute SQL Batches With JDBC and jOOQ                 jdbc
How to Execute SQL Batches With JDBC and jOOQ                 sql server
How to Execute SQL Batches With JDBC and jOOQ                 sql
How to Emulate Partial Indexes in Oracle                      optimisation
How to Emulate Partial Indexes in Oracle                      index
How to Emulate Partial Indexes in Oracle                      partial index
How to Emulate Partial Indexes in Oracle                      oracle
How to Emulate Partial Indexes in Oracle                      sql
How to Emulate Partial Indexes in Oracle                      postgresql
How to Emulate Partial Indexes in Oracle                      t-sql
How to Emulate Partial Indexes in Oracle                      sql server

You can immediately see the cross join semantics here, as we’re combining each tag (per post) with its post.

Looking for ordinals (i.e. the tag number inside of the array) along with the array? Easy!

Just add the powerful WITH ORDINALITY clause after the UNNEST() call in PostgreSQL:

SELECT title, tag
FROM blogs, LATERAL unnest(tags) WITH ORDINALITY AS tags(tag);

A bit more complicated to emulate in Oracle:

-- Fancy, with a window function
SELECT title, tags.*
FROM blogs, LATERAL (
  SELECT tags.*, ROW_NUMBER() OVER (ORDER BY NULL)
  FROM TABLE(tags) tags
) tags;

-- Classic, with ROWNUM
SELECT title, tags.*
FROM blogs, LATERAL (
  SELECT tags.*, ROWNUM
  FROM TABLE(tags) tags
) tags;

The result now contains the tag “ID”, i.e the ordinal of the tag inside of the array:

title                                           tag               ordinal
-------------------------------------------------------------------------
How to Fetch ... Cursors with JDBC and jOOQ     implicit cursor   1
How to Fetch ... Cursors with JDBC and jOOQ     batch             2
How to Fetch ... Cursors with JDBC and jOOQ     oracle            3
How to Fetch ... Cursors with JDBC and jOOQ     jooq              4
How to Fetch ... Cursors with JDBC and jOOQ     jdbc              5
How to Fetch ... Cursors with JDBC and jOOQ     resultset         6
How to Execute SQL Batches With JDBC and jOOQ   batch             1
How to Execute SQL Batches With JDBC and jOOQ   batch statement   2
How to Execute SQL Batches With JDBC and jOOQ   mysql             3
How to Execute SQL Batches With JDBC and jOOQ   jooq              4
How to Execute SQL Batches With JDBC and jOOQ   jdbc              5
How to Execute SQL Batches With JDBC and jOOQ   sql server        6
How to Execute SQL Batches With JDBC and jOOQ   sql               7
How to Emulate Partial Indexes in Oracle        optimisation      1
How to Emulate Partial Indexes in Oracle        index             2
How to Emulate Partial Indexes in Oracle        partial index     3
How to Emulate Partial Indexes in Oracle        oracle            4
How to Emulate Partial Indexes in Oracle        sql               5
How to Emulate Partial Indexes in Oracle        postgresql        6
How to Emulate Partial Indexes in Oracle        t-sql             7
How to Emulate Partial Indexes in Oracle        sql server        8

Now, imagine looking for those blog posts that are tagged “jooq”. Easy!

PostgreSQL:

SELECT title
FROM blogs
WHERE 'jooq' = ANY(tags);

Oracle:

SELECT title
FROM blogs
WHERE 'jooq' IN (SELECT * FROM TABLE(tags));

Yielding:

title
-----------------------------------------------------------
How to Fetch Oracle 12c Implicit Cursors with JDBC and jOOQ
How to Execute SQL Batches With JDBC and jOOQ

Conclusion

These are just a few nice things we can do when we denormalise our data into nested collections / arrays, and then use features like UNNEST to bring them back to the table level. Both Oracle and PostgreSQL support a variety of really nice features building on top of arrays, so do check them out!

2 thoughts on “Beautiful SQL: Lateral Unnesting of Array Columns

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