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SQL

Learn about SQL's history, types, syntax, and important commands such as SELECT, JOIN, WHERE, and more. Master SQL for data manipulation and retrieval.

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SQL

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  1. SQL Chapter 7 Most important: 7.2

  2. Structured Query Language • Started as "Sequel" • Now an ANSI (X3H2) / ISO standard • SQL2 or SQL-92 • SQL3 is in the works • Lots of uses and versions • As a direct language our focus • As an embedded language in a program in a conventional language • As a protocol from client to DB server increasingly important

  3. SQL DDL • TABLE/SCHEMA/CATALOG • CREATE/DROP/ALTER • SQL DATA TYPES • DB files are often character-based, descended from COBOL • INT, FLOAT, etc. • DEC(i,j) • CHAR(n), VARCHAR(n) • DATE/TIME

  4. 90% of what you need to know SELECT <attributes> FROM <tables> WHERE <conditions> • SELECT is a projection into the output • FROM is effectively a Cartesian product of all the tables used • WHERE are conditions, which could be very elaborate and even involve nested SELECTs.

  5. Some Notation • Attributes can be qualified • Tables can be renamed SELECT S.FNAME, S.LNAME FROM EMPLOYEE E, EMPLOYEE S WHERE E.SUPERSSN = S.ESSN • SELECT * means display all columns • No WHERE means display all rows

  6. More SQL Topics • Conditionals • NOT, AND, OR (in precedence order) • Tables vs sets • Duplicates are NOT eliminated on most operations! • DISTINCT keyword eliminates dups in SELECT

  7. Set Operations • UNION, intersections ("INTERSECT"), difference ("EXCEPT") • DO eliminate duplicates • Use to connect whole sets (result of queries), not within WHERE • EXCEPT not supported by MS Access 97 • Division is not an SQL operation • Older SQL had a CONTAINS (set inclusion)

  8. IN and EXISTS • IN • Tests for membership in a set: x  A • Binary operator returning Boolean value • used within conditions (WHERE) • left side a row (or value construed as a row) • right side a table, frequently the result of a (nested) SELECT • EXISTS • Unary operator returning a Boolean • tests a table for non-empty: A

  9. Aggregate Functions • Five standard functions: COUNT, SUM, MIN, MAX, AVG • Normally appears in SELECT clause SELECT SUM (SALARY) • Is applied after WHERE • Result is a column in the output • can sometimes think of as a scalar

  10. Grouping • GROUP BY <attributes> • causes rows with common values to be grouped together • no particular ordering among groups • Might be used just to improve output presentation, but… • Most commonly used in connection with aggregate functions

  11. Grouping and Functions In presence of grouping, aggregate functions apply to each group individually. • Output has one row per group • Order of processing: WHERE conditions, then GROUPING, then SELECT (including agg. functions). • But… what if you want a condition applied after the grouping??

  12. HAVING • Applies a condition after grouping • condition is applied to each group • May use aggregate functions • HAVING SUM(QUANTITY) > 350

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