SQL problems requiring cursors

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A cursor is a construct available in most implementations of SQL that allows the programmer to handle data in a row-by-row manner rather than as a group. Parallelizing row-by-row processing is much more complex than serial processing, which is another reason to make use of non-procedural SQL wherever possible. Database vendors typically handle parallel processing without requiring special handling by application developers.

Parallel processing can be orders of magnitude faster than serial processing.

Constraints[edit]

In this article, the following constraints apply:

  • The term "cursor" includes all cursor-like behavior. For example, using a loop in a shell script that loops across single-row SQL queries or the output of multi-row SQL queries is cursor-like behavior and does not achieve the goal of true set-based processing within the database.
  • All set-based SQL must be ANSI SQL. A number of vendors provide some extremely powerful proprietary extensions. The goal is to avoid such extensions in favor of ANSI SQL.
  • The solution must be generalizable. In one or more examples below, specific values may be used for demonstration purposes, but any solution must scale to any number feasible within the power of the database software and machine resources.

Example: Insert rows based on a count in the table itself[edit]

The table below represents types of marbles. The four text columns represent four marble characteristics. Each characteristic has two values for a total of 16 types of marbles.

The "quantity" column represents how many marbles of that type we have. The task is to create a second table holding one row for each marble of that type. Thus, the target table would have the four text columns, and a total of 40 + 20 + 20 + 10 + ... + 10 + 5 = 270 rows.

Source table:

  QUANTITY TEXTURE    APPEARANCE SHAPE      COLOR
---------- ---------- ---------- ---------- -----
        40 smooth     shiny      round      blue
        20 smooth     shiny      warped     blue
        20 smooth     dull       round      blue
        10 smooth     dull       warped     blue
        20 rough      shiny      round      blue
        10 rough      shiny      warped     blue
        10 rough      dull       round      blue
         5 rough      dull       warped     blue
        40 rough      dull       warped     red
        20 rough      dull       round      red
        20 rough      shiny      warped     red
        10 rough      shiny      round      red
        20 smooth     dull       warped     red
        10 smooth     dull       round      red
        10 smooth     shiny      warped     red
         5 smooth     shiny      round      red

Table to generate:

TEXTURE    APPEARANCE SHAPE      COLOR
---------- ---------- ---------- -----
smooth     shiny      round      blue   -- 1
smooth     shiny      round      blue   -- 2
...                                     -- and so on
smooth     shiny      round      blue   -- 40
smooth     shiny      warped     blue   -- 1
smooth     shiny      warped     blue   -- 2
...                                     -- and so on
smooth     shiny      warped     blue   -- 20
...                                     -- and so on
smooth     shiny      round      red    -- 1
smooth     shiny      round      red    -- 2
smooth     shiny      round      red    -- 3
smooth     shiny      round      red    -- 4
smooth     shiny      round      red    -- 5

Solution in cursor form[edit]

Generating the target table with a cursor is fairly simple.

DECLARE
  cursor c IS SELECT * FROM marbles_seed;
 
BEGIN
  FOR r IN c loop
    FOR i IN 1..r.quantity loop
      INSERT INTO marbles VALUES (
        r.texture,
        r.appearance,
        r.shape,
        r.color_actual
      );
    END loop;
  END loop;
END;

Solution in SQL[edit]

Solving the problem with SQL is a bit more code and requires a bit more creative thought than the nested loop approach of cursors.

Number table[edit]

The solution requires an intermediate table. The table has one column of type NUMBER that has the values 0 to whatever number of rows is needed. For this discussion, we'll limit it to one million rows. The code is as follows: Setup:

CREATE TABLE numbers_seed ( n NUMBER(1) );
CREATE TABLE numbers ( n NUMBER(7));
INSERT INTO numbers_seed VALUES ( 0 );
INSERT INTO numbers_seed VALUES ( 1 );
INSERT INTO numbers_seed VALUES ( 2 );
INSERT INTO numbers_seed VALUES ( 3 );
INSERT INTO numbers_seed VALUES ( 4 );
INSERT INTO numbers_seed VALUES ( 5 );
INSERT INTO numbers_seed VALUES ( 6 );
INSERT INTO numbers_seed VALUES ( 7 );
INSERT INTO numbers_seed VALUES ( 8 );
INSERT INTO numbers_seed VALUES ( 9 );
INSERT INTO numbers
SELECT n6.n * 100000 +
       n5.n * 10000 +
       n4.n * 1000 +
       n3.n * 100 +
       n2.n * 10 +
       n1.n * 1 n
FROM numbers_seed n1,
       numbers_seed n2,
       numbers_seed n3,
       numbers_seed n4,
       numbers_seed n5,
       numbers_seed n6

The numbers table can be created in parallel.

Solution core[edit]

Assume the marble type table above is named marbles_seed and the target table is named marbles. The code that generates the needed 270 rows is:

INSERT INTO marbles
(m.texture, m.appearance, m.shape, m.color_actual)
 
SELECT m.texture,
       m.appearance,
       m.shape,
       m.color_actual
  FROM marbles_seed m,
       numbers n
 WHERE m.quantity > n.n

The database can process this insert in parallel without the programmer's involvement.