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Category Archives: SQL
¨ Can also perform grouping on a relation before computing aggregates 🞑 Specify a GROUP BY A1,A2,... clause at end of query ¨ …
Can count individual attribute values COUNT(branch_name) COUNT(DISTINCT branch_name) ¨ Can also count the total number of tuples COUNT(*) 🞑 If used with grouping,…
¨ Sometimes need to eliminate duplicates in SQL queries 🞑 Can use DISTINCT keyword to eliminate duplicates ¨ Example: “Find the number of…
¨ This query produces an error: SELECT branch_name, MAX(amount) AS max_amt FROM loan; ¨ Aggregate functions compute a single value from a multiset…
🞑 Find average balance of accounts at Perryridge branch SELECT AVG(balance) FROM account WHERE branch_name = 'Perryridge'; ¨ Find maximum amount of any…
¨ SQL provides grouping and aggregate operations, just like relational algebra Aggregate functions: SUM sums the values in the collection AVG computes average…
Say ASC or DESC after attribute name to specify order 🞑 ASC is redundant, but can improve readability in some cases Can list…
¨ Find bank accounts with a balance under $700: SELECT account_number, balance FROM account WHERE balance < 700; ¨ Order results in increasing…
¨ SQL query results can be ordered by particular attributes Two main categories of query results: 🞑 “Not ordered by anything” Tuples can…
¨ SQL queries use the SELECT statement General form is: SELECT A1, A2, ... FROM r1, r2, ... WHERE P; 🞑 ri are…
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