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Predicates I page 2

Logical operators as arithmetic ones are being estimated in accordance with their seniority.

One-place operation NOT has highest priority. We can assure ourselves if we'll execute the two following queries.

Console
Execute
  1. -- models which are not PC;
  2. -- the second predicate is useless because it is entering condition that yet taken into account
  3. -- by the first predicate
  4. SELECT maker, model, type
  5. FROM Product
  6. WHERE NOT type='PC' OR type='Printer';

Console
Execute
  1. -- models of maker A, which are not PC
  2. SELECT maker, model, type
  3. FROM Product
  4. WHERE NOT type='PC' AND maker='A';

Parentheses can change the order of estimation of logical operators:

Console
Execute
  1. -- models that are not PC or printer, i.e. models of laptops in our case
  2. SELECT maker, model, type
  3. FROM Product
  4. WHERE NOT (type='PC' OR type='Printer');

Console
Execute
  1. -- models that are not PC produced by maker A
  2. SELECT maker, model, type
  3. FROM Product
  4. WHERE NOT (type='PC' AND maker='A');

Next priority has AND operator. Compare results of the following queries.

Console
Execute
  1. -- PC models produced by maker A and models of any type belonging to maker B
  2. SELECT maker, model, type
  3. FROM Product
  4. WHERE type='PC' AND maker='A' OR maker='B';

Console
Execute
  1. -- PC models produced by maker A or maker B
  2. SELECT maker, model, type
  3. FROM Product
  4. WHERE type='PC' AND (maker='A' OR maker='B');

Notes:

If you are afraid to forget the order in which logical operators are being estimated, use the parentheses.

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