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Add LATERAL Joins or CROSS APPLY to Your SQL Tool Chain

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Add LATERAL Joins or CROSS APPLY to Your SQL Tool Chain

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The T-SQL dialect has known the powerful CROSS APPLY and OUTER APPLY JOIN syntaxes for ages. The SQL:1999 standard had introduced almost equivalent “lateral derived tables”, which are finally supported with PostgreSQL 9.3, or Oracle 12c, which has adopted both the SQL standard LATERAL syntax and the T-SQL vendor-specific CROSS APPLY and OUTER APPLY syntaxes.

But what are we even talking about?

SQL features have a unique trait that few other languages have. They are obscure to those who don’t know them, as every language feature introduces a new syntax with new keywords. In this case: APPLY and LATERAL. But it really isn’t so hard to understand. All you do with a CROSS APPLY is a CROSS JOIN between two tables where the right-hand side of the join expression can reference columns from the left-hand side of the join expression. Consider the following example by Martin Smith on Stack Overflow:

Reusing column aliases

SELECT number,
       doubled_number,
       doubled_number_plus_one
FROM master..spt_values
CROSS APPLY (
  SELECT 2 * CAST(number AS BIGINT)
) CA1(doubled_number)
CROSS APPLY (
  SELECT doubled_number + 1
) CA2(doubled_number_plus_one)

See a SQLFiddle of the above example

In this example, we’re selecting numbers from a system table and cross apply a scalar subselect multiplying each number by two. Then to the whole table product, we cross apply another scalar subselect, adding one to the last number.

This particular example could also be implemented using subqueries in the SELECT clause. But as you can see in the above example, doubled_number_plus_one can be calculated from a previously calculated column in one go. That wouldn’t be so “simple” with subqueries.

Applying table-valued functions to each record

SELECT *
FROM sys.dm_exec_query_stats AS qs
CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle)

This example may be even more useful when you want to join a table-valued function to each record of another table.

PostgreSQL’s LATERAL derived tabels

In PostgreSQL, this can be done somewhat magically by put-ting table-valued functions in the SELECT clause:

SELECT x, GENERATE_SERIES(0, x)
FROM (VALUES(0), (1), (2)) t(x)

See a SQLFiddle of the above example

The above yields

| X | GENERATE_SERIES |
|---|-----------------|
| 0 |               0 |
| 1 |               0 |
| 1 |               1 |
| 2 |               0 |
| 2 |               1 |
| 2 |               2 |

Alternatively, since PostgreSQL 9.3, you can use an explicit lateral derived table as such:

SELECT x, y
FROM (VALUES(0), (1), (2)) t(x),
LATERAL GENERATE_SERIES(0, t.x) u(y)

See a SQLFiddle of the above example

Yielding again

| X | Y |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 1 | 1 |
| 2 | 0 |
| 2 | 1 |
| 2 | 2 |

CROSS APPLY and OUTER APPLY in jOOQ 3.3

The above clauses will also be supported in the upcoming editions of jOOQ 3.3 where you can write queries like this one here:

DSL.using(configuration)
   .select()
   .from(AUTHOR)
   .crossApply(
        select(count().as("c"))
       .from(BOOK)
       .where(BOOK.AUTHOR_ID.eq(AUTHOR.ID)))
   .fetch();

Or lateral joins:

DSL.using(configuration)
   .select()
   .from(
        values(row(0), row(1), row(2))
            .as("t", "x"),
        lateral(generateSeries(0,
                fieldByName("t", "x"))
            .as("u", "y")))
   .fetch();

No matter if you’re using jOOQ or native SQL, lateral derived tables or CROSS APPLY should definitely be part of your awesome SQL tool chain!



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Published at DZone with permission of Lukas Eder, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

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