Sql cheat-sheet
- 1. SQL cheat sheet Basic Queries Views -- filter your columns SELECT col1, col2, col3, ... FROM table1 -- filter the rows WHERE col4 = 1 AND col5 = 2 -- aggregate the data GROUP by … -- limit aggregated data HAVING count(*) > 1 -- order of the results ORDER BY col2 Useful keywords for SELECTS: DISTINCT - return unique results BETWEEN a AND b - limit the range, the values can be numbers, text, or dates LIKE - pattern search within the column text IN (a, b, c) - check if the value is contained among given. Data Modification -- update specific data with the WHERE clause UPDATE table1 SET col1 = 1 WHERE col2 = 2 -- insert values manually INSERT INTO table1 (ID, FIRST_NAME, LAST_NAME) VALUES (1, ‘Rebel’, ‘Labs’); -- or by using the results of a query INSERT INTO table1 (ID, FIRST_NAME, LAST_NAME) SELECT id, last_name, first_name FROM table2 A VIEW is a virtual table, which is a result of a query. They can be used to create virtual tables of complex queries. CREATE VIEW view1 AS SELECT col1, col2 FROM table1 WHERE … The Joy of JOINs A B A B RIGHT OUTER JOIN - all rows from table B, even if they do not exist in table A INNER JOIN - fetch the results that exist in both tables LEFT OUTER JOIN - all rows from table A, even if they do not exist in table B Updates on JOINed Queries You can use JOINs in your UPDATEs UPDATE t1 SET a = 1 FROM table1 t1 JOIN table2 t2 ON t1.id = t2.t1_id WHERE t1.col1 = 0 AND t2.col2 IS NULL; NB! Use database specific syntax, it might be faster! Semi JOINs You can use subqueries instead of JOINs: SELECT col1, col2 FROM table1 WHERE id IN (SELECT t1_id FROM table2 WHERE date > CURRENT_TIMESTAMP) Indexes If you query by a column, index it! CREATE INDEX index1 ON table1 (col1) Don’t forget: Avoid overlapping indexes Avoid indexing on too many columns Indexes can speed up DELETE and UPDATE operations Reporting Use aggregation functions COUNT - return the number of rows SUM - cumulate the values AVG - return the average for the group MIN / MAX - smallest / largest value Useful Utility Functions -- convert strings to dates: TO_DATE (Oracle, PostgreSQL), STR_TO_DATE (MySQL) -- return the first non-NULL argument: COALESCE (col1, col2, “default value”) -- return current time: CURRENT_TIMESTAMP -- compute set operations on two result sets SELECT col1, col2 FROM table1 UNION / EXCEPT / INTERSECT SELECT col3, col4 FROM table2; Union - returns data from both queries Except - rows from the first query that are not present in the second query Intersect - rows that are returned from both queries