uni/year2/semester1/logseq-stuff/pages/SQL DML%3A SELECT.md

title:: SQL DML: SELECT

• #CT230 - Database Systems I
• Previous Topic: SQL DML Statement
• Next Topic: SQL SELECT: Working with Strings & Subqueries
• Relevant Slides:

• Notes on Syntax

  • While SQL is case insensitive, Linux ^^is not.^^
    • web1.cs.nuigalway.ie is a ^^linux server^^, so we need to be careful with table names in particular, as EMPLOYEE is ^^not the same thing^^ as employee.
  • Attribute names are separated by commas.
  • Strings are enclosed in quotes.
  • Numbers are not enclosed in quotes.
  • Usually, SQL keywords are capitalised while table & keyword names are mostly kept in lowercase unless using camelCase instead of snake_case.
  • Code should be organised horizontally & vertically, and not all written on one line.
  • Code blocks are separated by a semicolon.
  • Comments can be made using #, --, & /* */.

• CRUD Operations in DML

  • What are CRUD Operations? #card card-last-interval:: 7.44 card-repeats:: 3 card-ease-factor:: 2.08 card-next-schedule:: 2022-11-22T02:35:29.285Z card-last-reviewed:: 2022-11-14T16:35:29.286Z card-last-score:: 3
    • CRUD operations are the 4 basic functions that we may wish to perform on persistent data.
      • Create: insert a new tuple. INSERT
      • Read: retrieve some data. SELECT
      • Update: modify some data. Update
      • Delete: delete some data or a tuple. DELETE

  • Read: SELECT

    • The basic syntax for an SQL select query to read data consists of 6 clauses. #card card-last-interval:: 19.01 card-repeats:: 4 card-ease-factor:: 2.18 card-next-schedule:: 2022-12-03T20:06:00.414Z card-last-reviewed:: 2022-11-14T20:06:00.414Z card-last-score:: 3

      • SELECT [DISTINCT] <attribute list>
        FROM <table list>
        WHERE <condition>
        GROUP BY <group attributes>
        HAVING <group condition>
        ORDER BY <attribute list>
        

      • The order of these clauses ^^cannot be changed.^^
      • SELECT & FROM are ^^always required^^, other clauses are optional.

    • SELECT <attribute list> FROM <table list> WHERE <condition> #card

      card-last-interval:: 4 card-repeats:: 2 card-ease-factor:: 2.9 card-next-schedule:: 2022-11-26T13:37:49.433Z card-last-reviewed:: 2022-11-22T13:37:49.433Z card-last-score:: 5
      • <attribute list> is a list of attribute (column) names (separated by commas) whose values will be retrieved by the query.
      • <table list> is a list of table names (separated by commas) containing the attributes.
      • <condition> is a Boolean expression that identifies the tuples to be retrieved by the query.
        • For each tuple (row) in the table(s) which are part of the query:
          • the tuple is checked to see if the condition is true for this tuple.
            • If true, the tuple is part of the output.
            • If false, the tuple is not part of the output.
        • The comparison operators are:
          • = <= < > >= !=
          • Conditions can be compounded by use of Boolean AND, OR, and can be negated with NOT.
          • ^^Note:^^ In some versions of SQL, (e.g., in MS), the != operator is written as <>.

    • Calculated or Derived Fields #card

      card-last-interval:: 28.3 card-repeats:: 4 card-ease-factor:: 2.66 card-next-schedule:: 2022-12-15T16:48:00.918Z card-last-reviewed:: 2022-11-17T09:48:00.918Z card-last-score:: 5
      • We can specify an SQL expression in the SELECT clause which can involve numerical operations on numeric fields and counting operations on non-numeric fields.
      • e.g.,

        • SELECT ssn, salary/12 FROM employee;
          

    • Tidying Up the Output #card

      card-last-interval:: 19.01 card-repeats:: 4 card-ease-factor:: 2.18 card-next-schedule:: 2022-12-03T20:05:40.995Z card-last-reviewed:: 2022-11-14T20:05:40.995Z card-last-score:: 3
      • We can use keywords CAST, AS, & DECIMAL(x, y) to specify the total number of digits x and the number of digits after the decimal point y when we're working with real numbers.

        • SELECT ssn, CAST(salary/12.0 AS DECIMAL(8,2))
          FROM employee;
          

      • We can also use the keyword AS to rename output.

        • SELECT ssn, CAST(salary/12.0 AS DECIMAL(8,2))
          AS monthlySalary
          FROM employee;
          

    • Keyword DISTINCT #card

      card-last-interval:: 33.64 card-repeats:: 4 card-ease-factor:: 2.9 card-next-schedule:: 2022-12-21T00:50:05.938Z card-last-reviewed:: 2022-11-17T09:50:05.938Z card-last-score:: 5
      • The keyword DISTINCT automatically removes duplicates from the returned result set.
      • We should be careful of using with large result sets as it can be an expensive operation to perform (not a problem for our small examples).

      • SELECT DISTINCT salary
        FROM employee;
        

    • Selecting All Attribute Values for Selected Tuples #card

      card-last-interval:: 11.2 card-repeats:: 3 card-ease-factor:: 2.8 card-next-schedule:: 2022-11-25T20:36:38.405Z card-last-reviewed:: 2022-11-14T16:36:38.405Z card-last-score:: 5
      • To retrieve all attribute values of selected tuples, you do not have to explicitly list all the attribute names - instead, you can use SELECT *.

    • New Operators

      • BETWEEN #card card-last-interval:: 33.64 card-repeats:: 4 card-ease-factor:: 2.9 card-next-schedule:: 2022-12-18T11:07:12.625Z card-last-reviewed:: 2022-11-14T20:07:12.625Z card-last-score:: 5
        • The BETWEEN operator selects values within a given range. The values can be numbers, texts, or dates.

        • SELECT <column name(s)>
          FROM <table name>
          WHERE <column name(s)> BETWEEN <value 1> AND <value 2>;
          

      • IN #card card-last-interval:: 24.2 card-repeats:: 4 card-ease-factor:: 2.42 card-next-schedule:: 2022-12-11T13:48:17.496Z card-last-reviewed:: 2022-11-17T09:48:17.496Z card-last-score:: 3
        • The IN operator tests if a data value matches one of a list of values.
        • It is a shorthand for multiple OR conditions.

        • SELECT <column name(s)>
          FROM <table_name>
          WHERE <column_name> IN (<value1>, <value2>, ...); 
          
          -- OR --
          
          SELECT <column_name(s)>
          FROM <table_name>
          WHERE <column_name> IN (SELECT <statement>); 
          

      • LIKE #card card-last-interval:: 33.64 card-repeats:: 4 card-ease-factor:: 2.9 card-next-schedule:: 2022-12-18T11:07:20.197Z card-last-reviewed:: 2022-11-14T20:07:20.198Z card-last-score:: 5
        • Allows string comparison, when equality is too strict.
        • There are 2 wildcards, often used in conjunction with the LIKE operator:
          • The percent sign % represents zero, one, or multiple characters.
          • The underscore sign _ represents one, single character.

        • SELECT <column_name(s)>
          FROM <table_name>
          WHERE <column_name> LIKE <pattern>; 
          

      • IS NULL #card card-last-interval:: 33.64 card-repeats:: 4 card-ease-factor:: 2.9 card-next-schedule:: 2022-12-21T00:49:58.730Z card-last-reviewed:: 2022-11-17T09:49:58.730Z card-last-score:: 5
        • Allows an explicit search for NULL.

    • Set Operators

      • UNION
      • INTERSECTION
      • MINUS / DIFFERENCE

• SELECT full_name, ssn FROM (
      SELECT ssn, CONCAT(fname, " ", minit, " ", lname)
      AS full_name from employee) AS dtable
  WHERE full_name IN ("John B Smith", "James E Borg", "Joyce A English");