- relational database , Based on the relational model , Use relationship （ surface ） Store the data , At the same time, the integrity constraint is defined . Common relational database systems include ：Oracle、MySQL/MariaDB、SQL Server、PostgreSQL wait .
- SQL, Structured query language , Standard language for accessing and manipulating relational databases .SQL It's declarative , Is a set oriented programming language .
1、 Single table query
SELECT col1, col2 AS c2 -- Column alias FROM t; -- The basic query
SELECT * FROM t; -- Query all fields
SELECT col1, col2, … FROM t WHERE conditions; -- Filter conditions
SELECT col1, col2, … FROM t ORDER BY col1 ASC, col2 DESC; -- Sort
SELECT col1, col2, … FROM t ORDER BY col1 ASC, col2 DESC OFFSET m ROWS FETCH FIRST n ROWS ONLY; -- Limit the number of
LIMIT n OFFSET m; -- Non standard implementation
SELECT col1, col2, agg_fun() -- Aggregate functions FROM t GROUP BY col1, col2 -- Group summary
HAVING conditions; -- Filter after grouping
2、 Multiple table joins
SELECT t1.col1, t2.col2, … FROM t1 INNER JOIN t2 ON conditions; -- Internal connection SELECT t1.col1, t2.col2, … FROM t1 LEFT JOIN t2 ON conditions; -- Left connection
SELECT t1.col1, t2.col2, … FROM t1 RIGHT JOIN t2 ON conditions; -- The right connection
SELECT t1.col1, t2.col2, … FROM t1 FULL JOIN t2 ON conditions; -- Full connection
SELECT t1.col1, t2.col2, … FROM t1 CROSS JOIN t2 ON conditions;-- Cross connect
SELECT a.col1, b.col2, … FROM t1 a -- Table alias JOIN t1 b ON conditions; -- Self join
3、 Set operations
SELECT col1, col2, … FROM t1 UNION [ALL]
SELECT c1, c2, … FROM t2; -- Union operation
SELECT col1, col2, … FROM t1 INTERSECT SELECT c1, c2, … FROM t2; -- Intersection operation
SELECT col1, col2, … FROM t1 EXCEPT -- MINUS
SELECT c1, c2, … FROM t2; -- Subtraction operation
4、 Subquery
SELECT col1, (subquery) AS c2 -- Scalar subquery FROM t;
SELECT col1, col2, … -- Line sub query FROM t WHERE (col1, col2) = (subquery);
SELECT t1.col1, t2.c2, … FROM t1 JOIN (subquery) t2 -- Table sub query ON conditions;
SELECT t1.col1, t1.col2, … FROM t1 WHERE EXISTS ( -- Associated subquery SELECT 1FROM t2 WHERE t2.c1 = t1.col1);
5、 Data manipulation
INSERT INTO t(col1, col2, …) VALUES (val1, val2, …); -- Insert a single piece of data
INSERT INTO t(col1, col2, …) SELECT …; -- Insert query results
UPDATE t SET col1 = val1, col2 = val2 WHERE conditions; -- Update data
DELETE FROM t WHERE conditions; -- Delete data
MEGRE INTO t1 USING t2 ON (condition) WHEN MATCHED THEN UPDATE SET col1 = val1, col2 = val2, ... WHEN NOT MATCHED THEN INSERT (col1, col2, ...) VALUES (val1, val2, ...); -- Merge data
6、 Data definition
CREATE TABLE t ( col1 INT NOT NULL PRIMARY KEY, col2 VARCHAR(50) NOT NULL, col3 DATE ); -- Create table
CREATE TABLE t(col1, col2, …) SELECT … ; -- Create tables based on query results
ALTER TABLE t ADD col3 INT; -- Add fields
ALTER TABLE t RENAME COLUMN col1 TO c1;
ALTER TABLE t DROP COLUMN col1;
DROP TABLE t; -- Delete table
TRUNCATE TABLE t; -- Delete all data in the table
CREATE VIEW v AS SELECT …; -- Create view
DROP VIEW v; -- Delete view
CREATE [UNIQUE] INDEX idx ON t(col1, col2); -- Create index
DROP INDEX idx; -- Delete index
Extended content
• The query criteria include ：=、!=、<>、<、<=、>、>=、BETWEEN、IN、EXISTS、LIKE、AND、OR、NOT、IS [NOT] NULL、ANY、ALL
• Integrity constraints include ：PRIMARY KEY、NOT NULL、FOREIGN KEY、CHECK、UNIQUE、DEFAULT.
• Common aggregate functions ：AVG、COUNT、SUM、MIN、MAX.
