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COMMONLY USED QUERIES

  • QUERYING DATA FROM A TABLE

Query data in columns c1, c2 from a table

SELECT c1, c2 FROM t;

Query all rows and columns from a table

SELECT c1, c2 FROM t
WHERE condition

Query distinct rows from a table

SELECT DISTINCT c1 FROM t
WHERE condition;

Sort the result set in ascending or descending order

SELECT c1, c2 FROM t
ORDER BY c1 ASC [DESC];

Skip offset of rows and return the next n rows

SELECT c1, c2 FROM t
ORDER BY c1
LIMIT n OFFSET offset;

Group rows using an aggregate function

SELECT c1, aggregate(c2)
FROM t
GROUP BY c1;

Filter groups using HAVING clause

SELECT c1, aggregate(c2) FROM t
GROUP BY c1
HAVING condition;

  • QUERYING FROM MULTIPLE TABLES

Inner join t1 and t2: Select all records from t1 and t2, where the join condition is met.

SELECT c1, c2
FROM t1
INNER JOIN t2 ON condition;

Left join t1 and t2: Select all records from t1, along with records from t2 for which the join condition is met (if at all)

SELECT c1, c2
FROM t1
LEFT JOIN t2 ON condition;

Right join t1 and t2: Select all records from t2, along with records from t1 for which the join condition is met (if at all).

SELECT c1, c2
FROM t1
RIGHT JOIN t2 ON condition;

Perform full join: Select all records from t1 and t2, regardless of whether the join condition is met or not.

SELECT c1, c2
FROM t1
FULL JOIN t2 ON condition;

SELECT c1, c2
FROM t1
CROSS JOIN t2;

Another way to perform cross join

SELECT c1, c2
FROM t1, t2;

Join t1 to itself using INNER JOIN clause

SELECT c1, c2
FROM t1 A
INNER JOIN t2 B ON condition;

  • USING SQL OPERATORS

Combine rows from two queries

SELECT c1, c2 FROM t1
UNION [ALL]
SELECT c1, c2 FROM t2;

Return the intersection of two queries

SELECT c1, c2 FROM t1
INTERSECT
SELECT c1, c2 FROM t2;

Subtract a result set from another result set

SELECT c1, c2 FROM t1
MINUS
SELECT c1, c2 FROM t2;

Query rows using pattern matching %, _

SELECT c1, c2 FROM t1
WHERE c1 [NOT] LIKE pattern;

Query rows in a list

SELECT c1, c2 FROM t
WHERE c1 [NOT] IN value_list;

Query rows between two values

SELECT c1, c2 FROM t
WHERE c1 BETWEEN low AND high;

Check if values in a table is NULL or not

SELECT c1, c2 FROM t
WHERE c1 IS [NOT] NULL;

  • MANAGING TABLES

Create a new table with three columns

CREATE TABLE t (
id INT PRIMARY KEY,
name VARCHAR NOT NULL, price INT DEFAULT 0
);

Delete the table from the database

DROP TABLE t ;

Add a new column to the table

ALTER TABLE t ADD column;

Drop column c from the table

ALTER TABLE t DROP COLUMN c ;

Add a constraint

ALTER TABLE t ADD constraint;

Drop a constraint

ALTER TABLE t DROP constraint;

Rename a table from t1 to t2

ALTER TABLE r1 RENAME TO t2;

Rename column c1 to c2

ALTER TABLE t1 RENAME c1 TO c2 ;

Remove all data in a table

TRUNCATE TABLE t;

  • USING SQL CONSTRAINTS

Set c1 and c2 as a primary key

CREATE TABLE t(
  c1 INT, c2 INT, c3 VARCHAR,
  PRIMARY KEY (c1,c2)
);

Set c2 column as a foreign key

CREATE TABLE t1(
  c1 INT PRIMARY KEY,
  c2 INT,
  FOREIGN KEY (c2) REFERENCES t2(c2)
);

Make the values in c1 and c2 unique

CREATE TABLE t(
  c1 INT, c1 INT,
  UNIQUE(c2,c3)
);

Ensure c1 > 0 and values in c1 >= c2

CREATE TABLE t(
  c1 INT, c2 INT,
  CHECK(c1> 0 AND c1 >= c2)
);

Set values in c2 column not NULL

CREATE TABLE t(
  c1 INT PRIMARY KEY,
  c2 VARCHAR NOT NULL
);

  • MODIFYING DATA

Insert one row into a table

INSERT INTO t(column_list)
VALUES(value_list);

Insert multiple rows into a table

INSERT INTO t(column_list)
VALUES (value_list),
         (value_list), ....;

Insert rows from t2 into t1

INSERT INTO t1(column_list) SELECT column_list
FROM t2;

Update new value in the column c1 for all rows

UPDATE t
SET c1 = new_value;

Update values in the column c1, c2 that match the condition

UPDATE t
SET c1 = new_value,
    c2 = new_value
WHERE condition;

Delete all data in a table

DELETE FROM t;

Delete subset of rows in a table

DELETE FROM t
WHERE condition;

  • MANAGING VIEWS

Create a new view that consists of c1 and c2

CREATE VIEW v(c1,c2) AS
SELECT c1, c2
FROM t;

Create a new view with check option

CREATE VIEW v(c1,c2) AS
SELECT c1, c2
FROM t;
WITH [CASCADED | LOCAL] CHECK OPTION;

Create a recursive view

CREATE RECURSIVE VIEW v
AS
select-statement // anchor part
UNION [ALL]
select-statement; // recursive part

Create a temporary view

CREATE TEMPORARY VIEW v AS
SELECT c1, c2
FROM t;

Delete a view

DROP VIEW view_name

  • MANAGING INDEXES

Create an index on c1 and c2 of the table t

CREATE INDEX idx_name
ON t(c1,c2);

Create a unique index on c3, c4 of the table t

CREATE UNIQUE INDEX idx_name
ON t(c3,c4);

Drop an index

DROP INDEX idx_name;

  • SQL AGGREGATE FUNCTIONS

AVG   // returns the average of a list
COUNT // returns the number of elements of a list
SUM   // returns the total of a list
MAX   // returns the maximum value in a list
MIN   // returns the minimum value in a list

  • MANAGING TRIGGERS

Create or modify a trigger

CREATE OR MODIFY TRIGGER trigger_name
WHEN EVENT
ON table_name TRIGGER_TYPE
EXECUTE stored_procedure;
  • WHEN
BEFORE // invoke before the event occurs
AFTER  // invoke after the event occurs
  • EVENT
INSERT // invoke for INSERT
UPDATE // invoke for UPDATE
DELETE // invoke for DELETE
  • TRIGGER_TYPE
FOR EACH ROW
FOR EACH STATEMENT

Create a trigger invoked before a new row is inserted into the person table

CREATE TRIGGER before_insert_person
BEFORE INSERT
ON person FOR EACH ROW
EXECUTE stored_procedure;

Delete a specific trigger

DROP TRIGGER trigger_name