- Naming conventions
- Query syntax
- Create syntax
SQL style guide
You can use this set of guidelines, fork them or make your own - the key here is that you pick a style and stick to it. To suggest changes or fix bugs please open an issue or pull request on GitHub.
These guidelines are designed to be compatible with Joe Celko’s SQL Programming Style book to make adoption for teams who have already read that book easier. This guide is a little more opinionated in some areas and in others a little more relaxed. It is certainly more succinct where Celko’s book contains anecdotes and reasoning behind each rule as thoughtful prose.
It is easy to include this guide in Markdown format as a part of a project’s code base or reference it here for anyone on the project to freely read—much harder with a physical book.
SQL style guide by Simon Holywell is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License. Based on a work at http://www.sqlstyle.guide.
- Use consistent and descriptive identifiers and names.
- Make judicious use of white space and indentation to make code easier to read.
- Store ISO-8601 compliant time and date information
- Try to use only standard SQL functions instead of vendor specific functions for reasons of portability.
- Keep code succinct and devoid of redundant SQL—such as unnecessary quoting or
WHEREclauses that can otherwise be derived.
- Include comments in SQL code where necessary. Use the C style opening
*/where possible otherwise precede comments with
--and finish them with a new line.
SELECT file_hash -- stored ssdeep hash FROM file_system WHERE file_name = '.vimrc';
/* Updating the file record after writing to the file */ UPDATE file_system SET file_modified_date = '1980-02-22 13:19:01.00000', file_size = 209732 WHERE file_name = '.vimrc';
- CamelCase—it is difficult to scan quickly.
- Descriptive prefixes or Hungarian notation such as
- Plurals—use the more natural collective term where possible instead. For example
- Quoted identifiers—if you must use them then stick to SQL92 double quotes for portability (you may need to configure your SQL server to support this depending on vendor).
- Object oriented design principles should not be applied to SQL or database structures.
- Ensure the name is unique and does not exist as a reserved keyword.
- Keep the length to a maximum of 30 bytes—in practice this is 30 characters unless you are using multi-byte character set.
- Names must begin with a letter and may not end with an underscore.
- Only use letters, numbers and underscores in names.
- Avoid the use of multiple consecutive underscores—these can be hard to read.
- Use underscores where you would naturally include a space in the name (first
- Avoid abbreviations and if you have to use them make sure they are commonly understood.
SELECT first_name FROM staff;
- Use a collective name or, less ideally, a plural form. For example (in order of
- Do not prefix with
tblor any other such descriptive prefix or Hungarian notation.
- Never give a table the same name as one of its columns and vice versa.
- Avoid, where possible, concatenating two table names together to create the name
of a relationship table. Rather than
- Always use the singular name.
- Where possible avoid simply using
idas the primary identifier for the table.
- Do not add a column with the same name as its table and vice versa.
- Always use lowercase except where it may make sense not to such as proper nouns.
Aliasing or correlations
- Should relate in some way to the object or expression they are aliasing.
- As a rule of thumb the correlation name should be the first letter of each word in the object’s name.
- If there is already a correlation with the same name then append a number.
- Always include the
ASkeyword—makes it easier to read as it is explicit.
- For computed data (
AVG()) use the name you would give it were it a column defined in the schema.
SELECT first_name AS fn FROM staff AS s1 JOIN students AS s2 ON s2.mentor_id = s1.staff_num;
SELECT SUM(s.monitor_tally) AS monitor_total FROM staff AS s;
- The name must contain a verb.
- Do not prefix with
sp_or any other such descriptive prefix or Hungarian notation.
The following suffixes have a universal meaning ensuring the columns can be read and understood easily from SQL code. Use the correct suffix where appropriate.
_id—a unique identifier such as a column that is a primary key.
_status—flag value or some other status of any type such as
_total—the total or sum of a collection of values.
_num—denotes the field contains any kind of number.
_name—signifies a name such as
_seq—contains a contiguous sequence of values.
_date—denotes a column that contains the date of something.
_size—the size of something such as a file size or clothing.
_addr—an address for the record could be physical or intangible such as
Always use uppercase for the reserved keywords
It is best to avoid the abbreviated keywords and use the full length ones where
Do not use database server specific keywords where an ANSI SQL keyword already exists performing the same function. This helps to make code more portable.
SELECT model_num FROM phones AS p WHERE p.release_date > '2014-09-30';
To make the code easier to read it is important that the correct complement of spacing is used. Do not crowd code or remove natural language spaces.
Spaces should be used to line up the code so that the root keywords all end on the same character boundary. This forms a river down the middle making it easy for the readers eye to scan over the code and separate the keywords from the implementation detail. Rivers are bad in typography, but helpful here.
(SELECT f.species_name, AVG(f.height) AS average_height, AVG(f.diameter) AS average_diameter FROM flora AS f WHERE f.species_name = 'Banksia' OR f.species_name = 'Sheoak' OR f.species_name = 'Wattle' GROUP BY f.species_name, f.observation_date) UNION ALL (SELECT b.species_name, AVG(b.height) AS average_height, AVG(b.diameter) AS average_diameter FROM botanic_garden_flora AS b WHERE b.species_name = 'Banksia' OR b.species_name = 'Sheoak' OR b.species_name = 'Wattle' GROUP BY b.species_name, b.observation_date);
FROM, etc. are all right aligned while the actual column
names and implementation specific details are left aligned.
Although not exhaustive always include spaces:
- before and after equals (
- after commas (
- surrounding apostrophes (
') where not within parentheses or with a trailing comma or semicolon.
SELECT a.title, a.release_date, a.recording_date FROM albums AS a WHERE a.title = 'Charcoal Lane' OR a.title = 'The New Danger';
Always include newlines/vertical space:
- after semicolons to separate queries for easier reading
- after each keyword definition
- after a comma when separating multiple columns into logical groups
- to separate code into related sections, which helps to ease the readability of large chunks of code.
Keeping all the keywords aligned to the righthand side and the values left aligned creates a uniform gap down the middle of query. It makes it much easier to scan the query definition over quickly too.
INSERT INTO albums (title, release_date, recording_date) VALUES ('Charcoal Lane', '1990-01-01 01:01:01.00000', '1990-01-01 01:01:01.00000'), ('The New Danger', '2008-01-01 01:01:01.00000', '1990-01-01 01:01:01.00000');
UPDATE albums SET release_date = '1990-01-01 01:01:01.00000' WHERE title = 'The New Danger';
SELECT a.title, a.release_date, a.recording_date, a.production_date -- grouped dates together FROM albums AS a WHERE a.title = 'Charcoal Lane' OR a.title = 'The New Danger';
To ensure that SQL is readable it is important that standards of indentation are followed.
Joins should be indented to the other side of the river and grouped with a new line where necessary.
SELECT r.last_name FROM riders AS r INNER JOIN bikes AS b ON r.bike_vin_num = b.vin_num AND b.engine_tally > 2 INNER JOIN crew AS c ON r.crew_chief_last_name = c.last_name AND c.chief = 'Y';
Subqueries should also be aligned to the right side of the river and then laid out using the same style as any other query. Sometimes it will make sense to have the closing parenthesis on a new line at the same character position as its opening partner—this is especially true where you have nested subqueries.
SELECT r.last_name, (SELECT MAX(YEAR(championship_date)) FROM champions AS c WHERE c.last_name = r.last_name AND c.confirmed = 'Y') AS last_championship_year FROM riders AS r WHERE r.last_name IN (SELECT c.last_name FROM champions AS c WHERE YEAR(championship_date) > '2008' AND c.confirmed = 'Y');
- Make use of
BETWEENwhere possible instead of combining multiple statements with
- Similarly use
IN()instead of multiple
- Where a value needs to be interpreted before leaving the database use the
CASEstatements can be nested to form more complex logical structures.
- Avoid the use of
UNIONclauses and temporary tables where possible. If the schema can be optimised to remove the reliance on these features then it most likely should be.
SELECT CASE postcode WHEN 'BN1' THEN 'Brighton' WHEN 'EH1' THEN 'Edinburgh' END AS city FROM office_locations WHERE country = 'United Kingdom' AND opening_time BETWEEN 8 AND 9 AND postcode IN ('EH1', 'BN1', 'NN1', 'KW1');
When declaring schema information it is also important to maintain human readable code. To facilitate this ensure the column definitions are ordered and grouped where it makes sense to do so.
Indent column definitions by four (4) spaces within the
Choosing data types
- Where possible do not use vendor specific data types—these are not portable and may not be available in older versions of the same vendor’s software.
- Only use
FLOATtypes where it is strictly necessary for floating point mathematics otherwise prefer
DECIMALat all times. Floating point rounding errors are a nuisance!
Specifying default values
- The default value must be the same type as the column—if a column is declared
DECIMALdo not provide an
- Default values must follow the data type declaration and come before any
Constraints and keys
Constraints and their subset, keys, are a very important component of any database definition. They can quickly become very difficult to read and reason about though so it is important that a standard set of guidelines are followed.
Deciding the column(s) that will form the keys in the definition should be a carefully considered activity as it will effect performance and data integrity.
- The key should be unique to some degree.
- Consistency in terms of data type for the value across the schema and a lower likelihood of this changing in the future.
- Can the value be validated against a standard format (such as one published by ISO)? Encouraging conformity to point 2.
- Keeping the key as simple as possible whilst not being scared to use compound keys where necessary.
It is a reasoned and considered balancing act to be performed at the definition of a database. Should requirements evolve in the future it is possible to make changes to the definitions to keep them up to date.
Once the keys are decided it is possible to define them in the system using constraints along with field value validation.
- Tables must have at least one key to be complete and useful.
- Constraints should be given a custom name excepting
FOREIGN KEYwhere the database vendor will generally supply sufficiently intelligible names automatically.
Layout and order
- Specify the primary key first right after the
- Constraints should be defined directly beneath the column they correspond to. Indent the constraint so that it aligns to the right of the column name.
- If it is a multi-column constraint then consider putting it as close to both
column definitions as possible and where this is difficult as a last resort
include them at the end of the
- If it is a table level constraint that applies to the entire table then it should also appear at the end.
- Use alphabetical order where
ON DELETEcomes before
- If it make senses to do so align each aspect of the query on the same character
position. For example all
NOT NULLdefinitions could start at the same character position. This is not hard and fast, but it certainly makes the code much easier to scan and read.
SIMILAR TOconstraints to ensure the integrity of strings where the format is known.
- Where the ultimate range of a numerical value is known it must be written as a
CHECK()to prevent incorrect values entering the database or the silent truncation of data too large to fit the column definition. In the least it should check that the value is greater than zero in most cases.
CHECK()constraints should be kept in separate clauses to ease debugging.
CREATE TABLE staff ( PRIMARY KEY (staff_num), staff_num INT(5) NOT NULL, first_name VARCHAR(100) NOT NULL, pens_in_drawer INT(2) NOT NULL, CONSTRAINT pens_in_drawer_range CHECK(pens_in_drawer >= 1 AND pens_in_drawer < 100) );
Designs to avoid
- Object oriented design principles do not effectively translate to relational database designs—avoid this pitfall.
- Placing the value in one column and the units in another column. The column
should make the units self evident to prevent the requirement to combine
columns again later in the application. Use
CHECK()to ensure valid data is inserted into the column.
- EAV (Entity Attribute Value) tables—use a specialist product intended for handling such schema-less data instead.
- Splitting up data that should be in one table across many because of arbitrary
concerns such as time-based archiving or location in a multi-national
organisation. Later queries must then work across multiple tables with
UNIONrather than just simply querying one table.
Reserved keyword reference
A list of ANSI SQL (92, 99 and 2003), MySQL 3 to 5.x, PostgreSQL 8.1, MS SQL Server 2000, MS ODBC and Oracle 10.2 reserved keywords.
A ABORT ABS ABSOLUTE ACCESS ACTION ADA ADD ADMIN AFTER AGGREGATE ALIAS ALL ALLOCATE ALSO ALTER ALWAYS ANALYSE ANALYZE AND ANY ARE ARRAY AS ASC ASENSITIVE ASSERTION ASSIGNMENT ASYMMETRIC AT ATOMIC ATTRIBUTE ATTRIBUTES AUDIT AUTHORIZATION AUTO_INCREMENT AVG AVG_ROW_LENGTH BACKUP BACKWARD BEFORE BEGIN BERNOULLI BETWEEN BIGINT BINARY BIT BIT_LENGTH BITVAR BLOB BOOL BOOLEAN BOTH BREADTH BREAK BROWSE BULK BY C CACHE CALL CALLED CARDINALITY CASCADE CASCADED CASE CAST CATALOG CATALOG_NAME CEIL CEILING CHAIN CHANGE CHAR CHAR_LENGTH CHARACTER CHARACTER_LENGTH CHARACTER_SET_CATALOG CHARACTER_SET_NAME CHARACTER_SET_SCHEMA CHARACTERISTICS CHARACTERS CHECK CHECKED CHECKPOINT CHECKSUM CLASS CLASS_ORIGIN CLOB CLOSE CLUSTER CLUSTERED COALESCE COBOL COLLATE COLLATION COLLATION_CATALOG COLLATION_NAME COLLATION_SCHEMA COLLECT COLUMN COLUMN_NAME COLUMNS COMMAND_FUNCTION COMMAND_FUNCTION_CODE COMMENT COMMIT COMMITTED COMPLETION COMPRESS COMPUTE CONDITION CONDITION_NUMBER CONNECT CONNECTION CONNECTION_NAME CONSTRAINT CONSTRAINT_CATALOG CONSTRAINT_NAME CONSTRAINT_SCHEMA CONSTRAINTS CONSTRUCTOR CONTAINS CONTAINSTABLE CONTINUE CONVERSION CONVERT COPY CORR CORRESPONDING COUNT COVAR_POP COVAR_SAMP CREATE CREATEDB CREATEROLE CREATEUSER CROSS CSV CUBE CUME_DIST CURRENT CURRENT_DATE CURRENT_DEFAULT_TRANSFORM_GROUP CURRENT_PATH CURRENT_ROLE CURRENT_TIME CURRENT_TIMESTAMP CURRENT_TRANSFORM_GROUP_FOR_TYPE CURRENT_USER CURSOR CURSOR_NAME CYCLE DATA DATABASE DATABASES DATE DATETIME DATETIME_INTERVAL_CODE DATETIME_INTERVAL_PRECISION DAY DAY_HOUR DAY_MICROSECOND DAY_MINUTE DAY_SECOND DAYOFMONTH DAYOFWEEK DAYOFYEAR DBCC DEALLOCATE DEC DECIMAL DECLARE DEFAULT DEFAULTS DEFERRABLE DEFERRED DEFINED DEFINER DEGREE DELAY_KEY_WRITE DELAYED DELETE DELIMITER DELIMITERS DENSE_RANK DENY DEPTH DEREF DERIVED DESC DESCRIBE DESCRIPTOR DESTROY DESTRUCTOR DETERMINISTIC DIAGNOSTICS DICTIONARY DISABLE DISCONNECT DISK DISPATCH DISTINCT DISTINCTROW DISTRIBUTED DIV DO DOMAIN DOUBLE DROP DUAL DUMMY DUMP DYNAMIC DYNAMIC_FUNCTION DYNAMIC_FUNCTION_CODE EACH ELEMENT ELSE ELSEIF ENABLE ENCLOSED ENCODING ENCRYPTED END END-EXEC ENUM EQUALS ERRLVL ESCAPE ESCAPED EVERY EXCEPT EXCEPTION EXCLUDE EXCLUDING EXCLUSIVE EXEC EXECUTE EXISTING EXISTS EXIT EXP EXPLAIN EXTERNAL EXTRACT FALSE FETCH FIELDS FILE FILLFACTOR FILTER FINAL FIRST FLOAT FLOAT4 FLOAT8 FLOOR FLUSH FOLLOWING FOR FORCE FOREIGN FORTRAN FORWARD FOUND FREE FREETEXT FREETEXTTABLE FREEZE FROM FULL FULLTEXT FUNCTION FUSION G GENERAL GENERATED GET GLOBAL GO GOTO GRANT GRANTED GRANTS GREATEST GROUP GROUPING HANDLER HAVING HEADER HEAP HIERARCHY HIGH_PRIORITY HOLD HOLDLOCK HOST HOSTS HOUR HOUR_MICROSECOND HOUR_MINUTE HOUR_SECOND IDENTIFIED IDENTITY IDENTITY_INSERT IDENTITYCOL IF IGNORE ILIKE IMMEDIATE IMMUTABLE IMPLEMENTATION IMPLICIT IN INCLUDE INCLUDING INCREMENT INDEX INDICATOR INFILE INFIX INHERIT INHERITS INITIAL INITIALIZE INITIALLY INNER INOUT INPUT INSENSITIVE INSERT INSERT_ID INSTANCE INSTANTIABLE INSTEAD INT INT1 INT2 INT3 INT4 INT8 INTEGER INTERSECT INTERSECTION INTERVAL INTO INVOKER IS ISAM ISNULL ISOLATION ITERATE JOIN K KEY KEY_MEMBER KEY_TYPE KEYS KILL LANCOMPILER LANGUAGE LARGE LAST LAST_INSERT_ID LATERAL LEADING LEAST LEAVE LEFT LENGTH LESS LEVEL LIKE LIMIT LINENO LINES LISTEN LN LOAD LOCAL LOCALTIME LOCALTIMESTAMP LOCATION LOCATOR LOCK LOGIN LOGS LONG LONGBLOB LONGTEXT LOOP LOW_PRIORITY LOWER M MAP MATCH MATCHED MAX MAX_ROWS MAXEXTENTS MAXVALUE MEDIUMBLOB MEDIUMINT MEDIUMTEXT MEMBER MERGE MESSAGE_LENGTH MESSAGE_OCTET_LENGTH MESSAGE_TEXT METHOD MIDDLEINT MIN MIN_ROWS MINUS MINUTE MINUTE_MICROSECOND MINUTE_SECOND MINVALUE MLSLABEL MOD MODE MODIFIES MODIFY MODULE MONTH MONTHNAME MORE MOVE MULTISET MUMPS MYISAM NAME NAMES NATIONAL NATURAL NCHAR NCLOB NESTING NEW NEXT NO NO_WRITE_TO_BINLOG NOAUDIT NOCHECK NOCOMPRESS NOCREATEDB NOCREATEROLE NOCREATEUSER NOINHERIT NOLOGIN NONCLUSTERED NONE NORMALIZE NORMALIZED NOSUPERUSER NOT NOTHING NOTIFY NOTNULL NOWAIT NULL NULLABLE NULLIF NULLS NUMBER NUMERIC OBJECT OCTET_LENGTH OCTETS OF OFF OFFLINE OFFSET OFFSETS OIDS OLD ON ONLINE ONLY OPEN OPENDATASOURCE OPENQUERY OPENROWSET OPENXML OPERATION OPERATOR OPTIMIZE OPTION OPTIONALLY OPTIONS OR ORDER ORDERING ORDINALITY OTHERS OUT OUTER OUTFILE OUTPUT OVER OVERLAPS OVERLAY OVERRIDING OWNER PACK_KEYS PAD PARAMETER PARAMETER_MODE PARAMETER_NAME PARAMETER_ORDINAL_POSITION PARAMETER_SPECIFIC_CATALOG PARAMETER_SPECIFIC_NAME PARAMETER_SPECIFIC_SCHEMA PARAMETERS PARTIAL PARTITION PASCAL PASSWORD PATH PCTFREE PERCENT PERCENT_RANK PERCENTILE_CONT PERCENTILE_DISC PLACING PLAN PLI POSITION POSTFIX POWER PRECEDING PRECISION PREFIX PREORDER PREPARE PREPARED PRESERVE PRIMARY PRINT PRIOR PRIVILEGES PROC PROCEDURAL PROCEDURE PROCESS PROCESSLIST PUBLIC PURGE QUOTE RAID0 RAISERROR RANGE RANK RAW READ READS READTEXT REAL RECHECK RECONFIGURE RECURSIVE REF REFERENCES REFERENCING REGEXP REGR_AVGX REGR_AVGY REGR_COUNT REGR_INTERCEPT REGR_R2 REGR_SLOPE REGR_SXX REGR_SXY REGR_SYY REINDEX RELATIVE RELEASE RELOAD RENAME REPEAT REPEATABLE REPLACE REPLICATION REQUIRE RESET RESIGNAL RESOURCE RESTART RESTORE RESTRICT RESULT RETURN RETURNED_CARDINALITY RETURNED_LENGTH RETURNED_OCTET_LENGTH RETURNED_SQLSTATE RETURNS REVOKE RIGHT RLIKE ROLE ROLLBACK ROLLUP ROUTINE ROUTINE_CATALOG ROUTINE_NAME ROUTINE_SCHEMA ROW ROW_COUNT ROW_NUMBER ROWCOUNT ROWGUIDCOL ROWID ROWNUM ROWS RULE SAVE SAVEPOINT SCALE SCHEMA SCHEMA_NAME SCHEMAS SCOPE SCOPE_CATALOG SCOPE_NAME SCOPE_SCHEMA SCROLL SEARCH SECOND SECOND_MICROSECOND SECTION SECURITY SELECT SELF SENSITIVE SEPARATOR SEQUENCE SERIALIZABLE SERVER_NAME SESSION SESSION_USER SET SETOF SETS SETUSER SHARE SHOW SHUTDOWN SIGNAL SIMILAR SIMPLE SIZE SMALLINT SOME SONAME SOURCE SPACE SPATIAL SPECIFIC SPECIFIC_NAME SPECIFICTYPE SQL SQL_BIG_RESULT SQL_BIG_SELECTS SQL_BIG_TABLES SQL_CALC_FOUND_ROWS SQL_LOG_OFF SQL_LOG_UPDATE SQL_LOW_PRIORITY_UPDATES SQL_SELECT_LIMIT SQL_SMALL_RESULT SQL_WARNINGS SQLCA SQLCODE SQLERROR SQLEXCEPTION SQLSTATE SQLWARNING SQRT SSL STABLE START STARTING STATE STATEMENT STATIC STATISTICS STATUS STDDEV_POP STDDEV_SAMP STDIN STDOUT STORAGE STRAIGHT_JOIN STRICT STRING STRUCTURE STYLE SUBCLASS_ORIGIN SUBLIST SUBMULTISET SUBSTRING SUCCESSFUL SUM SUPERUSER SYMMETRIC SYNONYM SYSDATE SYSID SYSTEM SYSTEM_USER TABLE TABLE_NAME TABLES TABLESAMPLE TABLESPACE TEMP TEMPLATE TEMPORARY TERMINATE TERMINATED TEXT TEXTSIZE THAN THEN TIES TIME TIMESTAMP TIMEZONE_HOUR TIMEZONE_MINUTE TINYBLOB TINYINT TINYTEXT TO TOAST TOP TOP_LEVEL_COUNT TRAILING TRAN TRANSACTION TRANSACTION_ACTIVE TRANSACTIONS_COMMITTED TRANSACTIONS_ROLLED_BACK TRANSFORM TRANSFORMS TRANSLATE TRANSLATION TREAT TRIGGER TRIGGER_CATALOG TRIGGER_NAME TRIGGER_SCHEMA TRIM TRUE TRUNCATE TRUSTED TSEQUAL TYPE UESCAPE UID UNBOUNDED UNCOMMITTED UNDER UNDO UNENCRYPTED UNION UNIQUE UNKNOWN UNLISTEN UNLOCK UNNAMED UNNEST UNSIGNED UNTIL UPDATE UPDATETEXT UPPER USAGE USE USER USER_DEFINED_TYPE_CATALOG USER_DEFINED_TYPE_CODE USER_DEFINED_TYPE_NAME USER_DEFINED_TYPE_SCHEMA USING UTC_DATE UTC_TIME UTC_TIMESTAMP VACUUM VALID VALIDATE VALIDATOR VALUE VALUES VAR_POP VAR_SAMP VARBINARY VARCHAR VARCHAR2 VARCHARACTER VARIABLE VARIABLES VARYING VERBOSE VIEW VOLATILE WAITFOR WHEN WHENEVER WHERE WHILE WIDTH_BUCKET WINDOW WITH WITHIN WITHOUT WORK WRITE WRITETEXT X509 XOR YEAR YEAR_MONTH ZEROFILL ZONE
Column data types
These are some suggested column data types to use for maximum compatibility between database engines.
- Exact numeric types
- Approximate numeric types
- DOUBLE PRECISION