SQL
http://tatiyants.com/pev/ - Explain Visualiser
| Data Description Language (DDL) | Data Manipulation Language (DML) | Data Control Language (DCL) | Transaction Control Language (TCL) |
|---|---|---|---|
| CREATE/ALTER/DROP/TRUNCATE/RENAME | SELECT/INSERT/UPDATE/DELETE | GRANT/REVOKE | BEGIN/COMMIT/ROLLBACK/SAVEPOINT |
Query Summary
The key actions are:
SELECT [DISTINCT] ... FROM ...
UNION
[INNER|LEFT|RIGHT|OUTER] JOIN ... ON ...
WHERE ...
GROUP BY ...
HAVING ...
ORDER BY ... [ASC|DESC]
LIMIT ... OFFSET ...
INSERT INTO ... VALUES ...
UPDATE ... SET ... [WHERE ...]
DELETE ...
SELECT can be used with AS to rename a table or a column.
UNION adds the rows of different queries
HAVING allows aggregate functions. WHERE filters individual rows before the GROUP BY, while HAVING filters group rows created by the GROUP BY.
The order is in fact:
- FROM [UNION/JOIN]
- WHERE
- GROUP BY
- HAVING
- SELECT
- ORDER BY
- LIMIT
ORDER BY clause = e.g. ORDER BY x DESC, y NULLS FIRST
SQL:2008 LIMIT clause = OFFSET n ROWS FETCH FIRST m ROWS ONLY
Operations
We have the arithmetic operators +, -, *, /, and %.
Logical/Comparison operators include:
IS (NOT) NULL
UNIQUE?
=, <, >
<>, !=
NOT, AND, OR
BETWEEN ... AND ...
EXISTS
IN ...
LIKE ...
... ANY, ALL
BETWEEN can be numbers, texts, or dates. In the case of text it seems to use alphabetical ordering (Unicode considerations)?
EXISTS tests whether a subquery returns any values (boolean existence check).
IN can use a tuple of values ('George', 'Clooney'), or a subquery.
LIKE uses two wildcards % (0+ characters), and _ (exactly one character). For example, _r% would mean values with "r" in the 2nd position.
ANY and ALL can be used with an operator against a subquery, e.g. WHERE column = ANY (SELECT ...).
Aggregate Functions
We have aggregate functions that can act on values:
MIN
MAX
COUNT
AVG
SUM
Be aware, that the aggregate selects on the column, and will not select the entire row.
Misc.
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