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FROM & JOIN Clauses

The FROM clause specifies the source of the data on which the remainder of the query should operate. Logically, the FROM clause is where the query starts execution. The FROM clause can contain a single table, a combination of multiple tables that are joined together using JOIN clauses, or another SELECT query inside a subquery node. DuckDB also has an optional FROM-first syntax which enables you to also query without a SELECT statement.

Examples

-- select all columns from the table called "table_name"
SELECT * FROM table_name;
-- select all columns from the table using the FROM-first syntax
FROM table_name SELECT *;
-- select all columns using the FROM-first syntax and omitting the SELECT clause
FROM table_name;
-- select all columns from the table called "table_name" through an alias "tn"
SELECT tn.* FROM table_name tn;
-- select all columns from the table "table_name" in the schema "schema_name"
SELECT * FROM schema_name.table_name;
-- select the column "i" from the table function "range",
-- where the first column of the range function is renamed to "i"
SELECT t.i FROM range(100) AS t(i);
-- select all columns from the CSV file called "test.csv"
SELECT * FROM 'test.csv';
-- select all columns from a subquery
SELECT * FROM (SELECT * FROM table_name);
-- select the entire row of the table as a struct
SELECT t FROM t;
-- select the entire row of the subquery as a struct (i.e., a single column)
SELECT t FROM (SELECT unnest(generate_series(41, 43)) AS x, 'hello' AS y) t;
-- join two tables together
SELECT * FROM table_name JOIN other_table ON (table_name.key = other_table.key);
-- select a 10% sample from a table
SELECT * FROM table_name TABLESAMPLE 10%;
-- select a sample of 10 rows from a table
SELECT * FROM table_name TABLESAMPLE 10 ROWS;
-- use the FROM-first syntax with WHERE clause and aggregation
FROM range(100) AS t(i) SELECT sum(t.i) WHERE i % 2 = 0;

Joins

Joins are a fundamental relational operation used to connect two tables or relations horizontally. The relations are referred to as the left and right sides of the join based on how they are written in the join clause. Each result row has the columns from both relations.

A join uses a rule to match pairs of rows from each relation. Often this is a predicate, but there are other implied rules that may be specified.

Outer Joins

Rows that do not have any matches can still be returned if an OUTER join is specified. Outer joins can be one of:

  • LEFT (All rows from the left relation appear at least once)
  • RIGHT (All rows from the right relation appear at least once)
  • FULL (All rows from both relations appear at least once)

A join that is not OUTER is INNER (only rows that get paired are returned).

When an unpaired row is returned, the attributes from the other table are set to NULL.

Cross Product Joins

The simplest type of join is a CROSS JOIN. There are no conditions for this type of join, and it just returns all the possible pairs.

-- return all pairs of rows
SELECT a.*, b.* FROM a CROSS JOIN b;

Conditional Joins

Most joins are specified by a predicate that connects attributes from one side to attributes from the other side. The conditions can be explicitly specified using an ON clause with the join (clearer) or implied by the WHERE clause (old-fashioned).

We use the l_regions and the l_nations tables from the TPC-H schema:

CREATE TABLE l_regions (
    r_regionkey INTEGER NOT NULL PRIMARY KEY,
    r_name      CHAR(25) NOT NULL,
    r_comment   VARCHAR(152)
);

CREATE TABLE l_nations (
    n_nationkey INTEGER NOT NULL PRIMARY KEY,
    n_name      CHAR(25) NOT NULL,
    n_regionkey INTEGER NOT NULL,
    n_comment   VARCHAR(152),
    FOREIGN KEY (n_regionkey) REFERENCES l_regions(r_regionkey)
);
-- return the regions for the nations
SELECT n.*, r.*
FROM l_nations n JOIN l_regions r ON (n_regionkey = r_regionkey);

If the column names are the same and are required to be equal, then the simpler USING syntax can be used:

CREATE TABLE l_regions (regionkey INTEGER NOT NULL PRIMARY KEY,
                        name      CHAR(25) NOT NULL,
                        comment   VARCHAR(152));

CREATE TABLE l_nations (nationkey INTEGER NOT NULL PRIMARY KEY,
                        name      CHAR(25) NOT NULL,
                        regionkey INTEGER NOT NULL,
                        comment   VARCHAR(152),
                        FOREIGN KEY (regionkey) REFERENCES l_regions(regionkey));
-- return the regions for the nations
SELECT n.*, r.*
FROM l_nations n JOIN l_regions r USING (regionkey);

The expressions to not have to be equalities - any predicate can be used:

-- return the pairs of jobs where one ran longer but cost less
SELECT s1.t_id, s2.t_id 
FROM west s1, west s2
WHERE s1.time > s2.time 
  AND s1.cost < s2.cost;

Semi and Anti Joins

Semi joins return rows from the left table that have at least one match in the right table. Anti joins return rows from the left table that have no matches in the right table. When using a semi or anti join the result will never have more rows than the left hand side table. Semi and anti joins provide the same logic as (NOT) IN statements.

-- return a list of cars that have a valid region.
SELECT cars.name, cars.manufacturer 
FROM cars
SEMI JOIN region
       ON cars.region = region.id;
-- return a list of cars with no recorded safety data.
SELECT cars.name, cars.manufacturer
FROM cars
ANTI JOIN safety_data
       ON cars.safety_report_id = safety_data.report_id;

Lateral Joins

The LATERAL keyword allows subqueries in the FROM clause to refer to previous subqueries. This feature is also known as a lateral join.

SELECT *
FROM range(3) t(i), LATERAL (SELECT i + 1) t2(j);
┌───────┬───────┐
│   i   │   j   │
│ int64 │ int64 │
├───────┼───────┤
│     0 │     1 │
│     1 │     2 │
│     2 │     3 │
└───────┴───────┘

Lateral joins are a generalization of correlated subqueries, as they can return multiple values per input value rather than only a single value.

SELECT *
FROM 
    generate_series(0, 1) t(i),
    LATERAL (SELECT i + 10 UNION ALL SELECT i + 100) t2(j);
┌───────┬───────┐
│   i   │   j   │
│ int64 │ int64 │
├───────┼───────┤
│     0 │    10 │
│     1 │    11 │
│     0 │   100 │
│     1 │   101 │
└───────┴───────┘

It may be helpful to think about LATERAL as a loop where we iterate through the rows of the first subquery and use it as input to the second (LATERAL) subquery. In the examples above, we iterate through table t and refer to its column i from the definition of table t2. The rows of t2 form column j in the result.

It is possible to refer to multiple attributes from the LATERAL subquery. Using the table from the first example:

CREATE TABLE t1 AS SELECT * FROM range(3) t(i), LATERAL (SELECT i + 1) t2(j);
SELECT * FROM t1, LATERAL (SELECT i + j) t2(k) ORDER BY ALL;
┌───────┬───────┬───────┐
│   i   │   j   │   k   │
│ int64 │ int64 │ int64 │
├───────┼───────┼───────┤
│     0 │     1 │     1 │
│     1 │     2 │     3 │
│     2 │     3 │     5 │
└───────┴───────┴───────┘

DuckDB detects when LATERAL joins should be used, making the use of the LATERAL keyword optional.

Positional Joins

When working with data frames or other embedded tables of the same size, the rows may have a natural correspondence based on their physical order. In scripting languages, this is easily expressed using a loop:

for (i = 0; i < n; i++) {
    f(t1.a[i], t2.b[i])
}

It is difficult to express this in standard SQL because relational tables are not ordered, but imported tables (like data frames) or disk files (like CSVs or Parquet files) do have a natural ordering.

Connecting them using this ordering is called a positional join:

-- treat two data frames as a single table
SELECT df1.*, df2.*
FROM df1
POSITIONAL JOIN df2;

Positional joins are always FULL OUTER joins.

As-Of Joins

A common operation when working with temporal or similarly-ordered data is to find the nearest (first) event in a reference table (such as prices). This is called an as-of join:

-- attach prices to stock trades
SELECT t.*, p.price
FROM trades t
ASOF JOIN prices p 
       ON t.symbol = p.symbol AND t.when >= p.when;

The ASOF join requires at least one inequality condition on the ordering field. The inequality can be any inequality condition (>=, >, <=, <) on any data type, but the most common form is >= on a temporal type. Any other conditions must be equalities (or NOT DISTINCT). This means that the left/right order of the tables is significant.

ASOF joins each left side row with at most one right side row. It can be specified as an OUTER join to find unpaired rows (e.g., trades without prices or prices which have no trades.)

-- attach prices or NULLs to stock trades
SELECT *
FROM trades t
ASOF LEFT JOIN prices p 
            ON t.symbol = p.symbol AND t.when >= p.when;

ASOF joins can also specify join conditions on matching column names with the USING syntax, but the last attribute in the list must be the inequality, which will be greater than or equal to (>=):

SELECT *
FROM trades t
ASOF JOIN prices p USING (symbol, "when");
-- Returns symbol, trades.when, price (but NOT prices.when)

If you combine USING with a SELECT * like this, the query will return the left side (probe) column values for the matches, not the right side (build) column values. To get the prices times in the example, you will need to list the columns explicitly:

SELECT t.symbol, t.when AS trade_when, p.when AS price_when, price
FROM trades t
ASOF LEFT JOIN prices p USING (symbol, "when");

Syntax

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Last modified: 2024-03-02