Typical mistakes at the solution of SELECT statement exercises

Tips and solutions

For this simple task as an example, we can demonstrate a variety of solutions, which is due to flexibility of SQL.

Solution 4.4.4. External self-join  

Console

Execute

SELECT DISTINCT p.maker 
FROM Product p LEFT JOIN 
 Product p1 ON p.maker = p1.maker AND 
 p1.type = 'Laptop'
WHERE p.type = 'PC' AND 
  p1.maker IS NULL;

SELECT DISTINCT p.maker FROM Product p LEFT JOIN Product p1 ON p.maker = p1.maker AND p1.type = 'Laptop' WHERE p.type = 'PC' AND   p1.maker IS NULL;

    Scheme

Left self-join the Product table is provided if the manufacturer is the same, and the type of production from the second table is a portable computer. Then the column p1.maker will be NULL, if the vendor have no models of portable computers and is used in a WHERE clause predicate, along with the condition that in the same row the type of production is the PC.

Solution 4.4.5. Grouping

Console

Execute

SELECT maker
FROM (SELECT DISTINCT maker, type 
 FROM Product 
 WHERE type IN ('PC', 'Laptop')
 ) AS a
GROUP BY maker
HAVING COUNT(*) = 1 AND 
  MAX(type) = 'PC';

SELECT maker FROM (SELECT DISTINCT maker, type FROM Product WHERE type IN ('PC', 'Laptop') ) AS a GROUP BY maker HAVING COUNT(*) = 1 AND   MAX(type) = 'PC';

    Scheme

In the subquery, unique pairs (supplier, type) are selected if the type is PC or portable computer. Then, you group by the supplier, with the grouped rows must meet the following conditions:

COUNT(*) = 1 — that is, the supplier should produce only one type of product from the remaining (because we have already cut off the printers, it is either a PC or portable computer);

MAX(type) = 'PC' — this type of product is a PC. Since the HAVING clause can not contain links to columns without aggregate functions, it uses MAX (type), although with the same success could write and MIN (type).

With so many approaches the question of the effectiveness are raised, which of the submitted requests will be faster. The leader here, by the number of operations and by execution time is the solution 4.4.5. The worst performance is in the third variant. Others are inferior to the leader about twice the time.

If you have not  Cистема управления реляционными базами данных (СУБД), разработанная корпорацией Microsoft. SQL(Structured Query Language) is a database computer language designed for the retrieval and management of data in relational database management systems (RDBMS), database schema creation and modification, and database object access control management.SQL Server installed, you can get the query execution plan directly from the site.

To return to discussion of exercise #8

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