6.4.1 SELECT Syntax

SELECT [STRAIGHT_JOIN]
       [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
       [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS] [HIGH_PRIORITY]
       [DISTINCT | DISTINCTROW | ALL]
    select_expression,...
    [INTO {OUTFILE | DUMPFILE} 'file_name' export_options]
    [FROM table_references
      [WHERE where_definition]
      [GROUP BY {unsigned_integer | col_name | formula} [ASC | DESC], ...
        [WITH ROLLUP]]
      [HAVING where_definition]
      [ORDER BY {unsigned_integer | col_name | formula} [ASC | DESC] ,...]
      [LIMIT [offset,] row_count | row_count OFFSET offset]
      [PROCEDURE procedure_name(argument_list)]
      [FOR UPDATE | LOCK IN SHARE MODE]]

SELECT is used to retrieve rows selected from one or more tables. Each select_expression indicates a column you want to retrieve. SELECT may also be used to retrieve rows computed without reference to any table. For example:

mysql> SELECT 1 + 1;
         -> 2

All clauses used must be given in exactly the order shown above. For example, a HAVING clause must come after any GROUP BY clause and before any ORDER BY clause.

• A SELECT expression may be given an alias using AS alias_name. The alias is used as the expression's column name and can be used with ORDER BY or HAVING clauses. For example:

  mysql> SELECT CONCAT(last_name,', ',first_name) AS full_name
      FROM mytable ORDER BY full_name;
  

  The AS keyword is optional when aliasing a SELECT expression. The preceding example could have been written like this:

  mysql> SELECT CONCAT(last_name,', ',first_name) full_name
      FROM mytable ORDER BY full_name;
  

  Because the AS is optional, a subtle problem can occur if you forget the comma between two SELECT expressions: MySQL will interpret the second as an alias name. For example, in the following statement, columnb is treated as an alias name:

  mysql> SELECT columna columnb FROM mytable;
  

• It is not allowed to use a column alias in a WHERE clause, because the column value may not yet be determined when the WHERE clause is executed. See section A.5.4 Problems with alias.
• The FROM table_references clause indicates the tables from which to retrieve rows. If you name more than one table, you are performing a join. For information on join syntax, see section 6.4.1.1 JOIN Syntax. For each table specified, you may optionally specify an alias.

  table_name [[AS] alias] [[USE INDEX (key_list)] | [IGNORE INDEX (key_list)] | FORCE INDEX (key_list)]]
  

  As of MySQL Version 3.23.12, you can give hints about which index MySQL should use when retrieving information from a table. This is useful if EXPLAIN shows that MySQL is using the wrong index from the list of possible indexes. By specifying USE INDEX (key_list), you can tell MySQL to use only one of the possible indexes to find rows in the table. The alternative syntax IGNORE INDEX (key_list) can be used to tell MySQL to not use some particular index. In MySQL 4.0.9 you can also use FORCE INDEX. This acts likes USE INDEX (key_list) but with the addition that a table scan is assumed to be VERY expensive. In other words a table scan will only be used if there is no way to use one of the given index to find rows in the table. USE/IGNORE/FORCE KEY are synonyms for USE/IGNORE/FORCE INDEX. In MySQL 4.0.14 you can use SET MAX_SEEKS_FOR_KEY=# as an alternative way to force MySQL to prefer key scans instead of table scans.
• You can refer to a table as tbl_name (within the current database), or as dbname.tbl_name to explicitly specify a database. You can refer to a column as col_name, tbl_name.col_name, or db_name.tbl_name.col_name. You need not specify a tbl_name or db_name.tbl_name prefix for a column reference in a SELECT statement unless the reference would be ambiguous. See section 6.1.2 Database, Table, Index, Column, and Alias Names, for examples of ambiguity that require the more explicit column reference forms.
• From version 4.1.0, you are allowed to specify DUAL as a dummy table name, in situations where no tables are referenced. This is purely compatibility feature, some other servers require this syntax.

  mysql> SELECT 1 + 1 FROM DUAL;
           -> 2
  

• A table reference may be aliased using tbl_name [AS] alias_name:

  mysql> SELECT t1.name, t2.salary FROM employee AS t1, info AS t2
      ->        WHERE t1.name = t2.name;
  mysql> SELECT t1.name, t2.salary FROM employee t1, info t2
      ->        WHERE t1.name = t2.name;
  

• Columns selected for output may be referred to in ORDER BY and GROUP BY clauses using column names, column aliases, or column positions. Column positions begin with 1:

  mysql> SELECT college, region, seed FROM tournament
      ->        ORDER BY region, seed;
  mysql> SELECT college, region AS r, seed AS s FROM tournament
      ->        ORDER BY r, s;
  mysql> SELECT college, region, seed FROM tournament
      ->        ORDER BY 2, 3;
  

  To sort in reverse order, add the DESC (descending) keyword to the name of the column in the ORDER BY clause that you are sorting by. The default is ascending order; this may be specified explicitly using the ASC keyword.
• In the WHERE clause, you can use any of the functions that MySQL supports, except for aggregate (summary) functions. See section 6.3 Functions for Use in SELECT and WHERE Clauses.
• The HAVING clause can refer to any column or alias named in the select_expression. It is applied nearly last, just before items are sent to the client, with no optimisation. (LIMIT is applied after HAVING.) Don't use HAVING for items that should be in the WHERE clause. For example, do not write this:

  mysql> SELECT col_name FROM tbl_name HAVING col_name > 0;
  

  Write this instead:

  mysql> SELECT col_name FROM tbl_name WHERE col_name > 0;
  

  In MySQL Version 3.22.5 or later, you can also write queries like this:

  mysql> SELECT user,MAX(salary) FROM users
      ->        GROUP BY user HAVING MAX(salary)>10;
  

  In older MySQL versions, you can write this instead:

  mysql> SELECT user,MAX(salary) AS sum FROM users
      ->        group by user HAVING sum>10;
  

• The options DISTINCT, DISTINCTROW and ALL specify whether duplicate rows should be returned. The default is (ALL), all matching rows are returned. DISTINCT and DISTINCTROW are synonyms and specify that duplicate rows in the result set should be removed.
• STRAIGHT_JOIN, HIGH_PRIORITY, and options beginning with SQL_ are MySQL extensions to SQL-99.
  • STRAIGHT_JOIN forces the optimiser to join the tables in the order in which they are listed in the FROM clause. You can use this to speed up a query if the optimiser joins the tables in non-optimal order. See section 5.2.1 EXPLAIN Syntax (Get Information About a SELECT).
  • HIGH_PRIORITY will give the SELECT higher priority than a statement that updates a table. You should only use this for queries that are very fast and must be done at once. A SELECT HIGH_PRIORITY query will run if the table is locked for read even if there is an update statement that is waiting for the table to be free.
  • SQL_BIG_RESULT can be used with GROUP BY or DISTINCT to tell the optimiser that the result set will have many rows. In this case, MySQL will directly use disk-based temporary tables if needed. MySQL will also, in this case, prefer sorting to doing a temporary table with a key on the GROUP BY elements.
  • SQL_BUFFER_RESULT forces the result to be put into a temporary table. This helps MySQL free the table locks early and helps in cases where it takes a long time to send the result set to the client.
  • SQL_SMALL_RESULT, a MySQL-specific option, can be used with GROUP BY or DISTINCT to tell the optimiser that the result set will be small. In this case, MySQL uses fast temporary tables to store the resulting table instead of using sorting. In MySQL Version 3.23 this shouldn't normally be needed.
  • SQL_CALC_FOUND_ROWS (version 4.0.0 and up) tells MySQL to calculate how many rows there would be in the result set, disregarding any LIMIT clause. The number of rows can then be retrieved with SELECT FOUND_ROWS(). See section 6.3.6.2 Miscellaneous Functions. Please note that in versions prior to 4.1.0 this does not work with LIMIT 0, which is optimised to return instantly (resulting in a row count of 0). See section 5.2.9 How MySQL Optimises LIMIT.
  • SQL_CACHE tells MySQL to store the query result in the query cache if you are using QUERY_CACHE_TYPE=2 (DEMAND). See section 6.9 MySQL Query Cache. In case of query with UNIONs and/or subqueries this option will take effect to be used in any SELECT of the query.
  • SQL_NO_CACHE tells MySQL not to store the query result in the query cache. See section 6.9 MySQL Query Cache. In case of query with UNIONs and/or subqueries this option will take effect to be used in any SELECT of the query.
• If you use GROUP BY, the output rows will be sorted according to the GROUP BY as if you had an ORDER BY over all the fields in the GROUP BY. MySQL has extended the GROUP BY clause so that you can also specify ASC and DESC after columns named in the clause:

  SELECT a,COUNT(b) FROM test_table GROUP BY a DESC
  

• MySQL has extended the use of GROUP BY to allow you to select fields that are not mentioned in the GROUP BY clause. If you are not getting the results you expect from your query, please read the GROUP BY description. See section 6.3.7 Functions and Modifiers for Use with GROUP BY Clauses.
• As of MySQL 4.1.1, GROUP BY allows a WITH ROLLUP modifier. See section 6.3.7.2 GROUP BY Modifiers.
• The LIMIT clause can be used to constrain the number of rows returned by the SELECT statement. LIMIT takes one or two numeric arguments, which must be integer constants. With one argument, the value specifies the number of rows to return from the beginning of the result set. With two arguments, the first specifies the offset of the first row to return, the second specifies the maximum number of rows to return. The offset of the initial row is 0 (not 1): To be compatible with PostgreSQL MySQL also supports the syntax: LIMIT row_count OFFSET offset.

  mysql> SELECT * FROM table LIMIT 5,10;  # Retrieve rows 6-15
  

  To retrieve all rows from a certain offset up to the end of the result set, you can use -1 for the second parameter:

  mysql> SELECT * FROM table LIMIT 95,-1; # Retrieve rows 96-last.
  

  If one argument is given, it indicates the maximum number of rows to return:

  mysql> SELECT * FROM table LIMIT 5;     # Retrieve first 5 rows
  

  In other words, LIMIT n is equivalent to LIMIT 0,n.
• The SELECT ... INTO OUTFILE 'file_name' form of SELECT writes the selected rows to a file. The file is created on the server host and cannot already exist (among other things, this prevents database tables and files such as `/etc/passwd' from being destroyed). You must have the FILE privilege on the server host to use this form of SELECT. The SELECT ... INTO OUTFILE statement is intended primarily to let you very quickly dump a table on the server machine. If you want to create the resulting file on some other host than the server host, you can't use SELECT ... INTO OUTFILE. In this case you should instead use some client program like mysqldump --tab or mysql -e "SELECT ..." > outfile to generate the file. SELECT ... INTO OUTFILE is the complement of LOAD DATA INFILE; the syntax for the export_options part of the statement consists of the same FIELDS and LINES clauses that are used with the LOAD DATA INFILE statement. See section 6.4.9 LOAD DATA INFILE Syntax. In the resulting text file, only the following characters are escaped by the ESCAPED BY character:
  • The ESCAPED BY character
  • The first character in FIELDS TERMINATED BY
  • The first character in LINES TERMINATED BY
  Additionally, ASCII 0 is converted to ESCAPED BY followed by 0 (ASCII 48). The reason for the above is that you must escape any FIELDS TERMINATED BY, ESCAPED BY, or LINES TERMINATED BY characters to reliably be able to read the file back. ASCII 0 is escaped to make it easier to view with some pagers. As the resulting file doesn't have to conform to the SQL syntax, nothing else need be escaped. Here follows an example of getting a file in the format used by many old programs.

  SELECT a,b,a+b INTO OUTFILE "/tmp/result.text"
  FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"'
  LINES TERMINATED BY "\n"
  FROM test_table;
  

• If you use INTO DUMPFILE instead of INTO OUTFILE, MySQL will only write one row into the file, without any column or line terminations and without any escaping. This is useful if you want to store a BLOB value in a file.
• Note that any file created by INTO OUTFILE and INTO DUMPFILE will be writeable by all users on the server host! The reason is that the MySQL server can't create a file that is owned by anyone else than the user it's running as (you should never run mysqld as root). The file thus must be world-writeable so that you can manipulate its contents.
• A PROCEDURE clause names a procedure that should process the data in the result set. For an example, see section 12.3.1 Procedure Analyse.
• If you use FOR UPDATE on a storage engine with page or row locks, the examined rows are write locked until the end of the current transaction.

Subsections

• 6.4.1.1 JOIN Syntax
• 6.4.1.2 UNION Syntax

User Comments

You can simulate a CROSSTAB by the following method:-

Use IF function to select the key value of the sub table as in:

SELECT
SUM(IF(beta_idx=1, beta_value,0)) as beta1_value,
SUM(IF(beta_idx=2, beta_value,0)) as beta2_value,
SUM(IF(beta_idx=3, beta_value,0)) as beta3_value
FROM alpha JOIN beta WHERE alpha_id = beta_alpha_id;

where alpha table has the form alpha_id, alpha_blah, alpha_blah_blah
and beta table has the form beta_alpha_id, beta_other stuff,
beta_idx, beta_value

This will create 3 columns with totals of beta values according to their idx field

when selecting a single random row you have to use a query like this: SELECT ... FROM my_table ORDER BY RAND() LIMIT 1.
as explain shows, mysql optimizes this VERY badly (or may be better said, doens't optimize it at all): it uses an temporary table and an extra filesort.
couldn't this be optimized?!
if not, may be add a syntax like SELECT RANDOM_ROW .... FROM my_table ...

This method of selecting a random row should be fast:

LOCK TABLES foo READ;
SELECT FLOOR(RAND() * COUNT(*)) AS rand_row FROM foo;
SELECT * FROM foo LIMIT $rand_row, 1;
UNLOCK TABLES;

Unfortunately, variables cannot be used in the LIMIT clause, otherwise the entire thing could be done completely in SQL.

If you want to produce a CSV file of the data in a table, the following should accomplish it:

select INTO OUTFILE 'filename' fields terminated by ',' optionally enclosed by '"' escaped by '\\' from tablename;

Remember that the CSV file will be created on the box that mysql is running on. The user that mysqld is running as must have permission to create the file.

If you wish to convert the '\N' in the file which is output to some other string like 'NULL' the following perl script will do that:

#!/usr/bin/perl

while (<>) {
s/\\N/NULL/g;
print;
}

Why can't an outfile return the headers? They are far harder to create than to delete.

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