SELECT
Syntax
SELECT
is used to retrieve rows selected from one or more tables, and can include UNION
statements and subqueries. See , "UNION
Syntax", and , "Subquery Syntax".
The most commonly used clauses of SELECT
statements are these:
- Each
select_expr
indicates a column that you want to retrieve. There must be at least oneselect_expr
. table_references
indicates the table or tables from which to retrieve rows. Its syntax is described in , "JOIN
Syntax".- Starting in MariaDB 5.6.2,
SELECT
supports explicit partition selection using thePARTITION
with a list of partitions or subpartitions (or both) following the name of the table in atable_reference
(see , "JOIN
Syntax"). In this case, rows are selected only from the partitions listed, and any other partitions of the table are ignored. For more information and examples, see , "Partition Selection". - The
WHERE
clause, if given, indicates the condition or conditions that rows must satisfy to be selected.where_condition
is an expression that evaluates to true for each row to be selected. The statement selects all rows if there is noWHERE
clause.In the
WHERE
expression, you can use any of the functions and operators that MariaDB supports, except for aggregate (summary) functions. See , "Expression Syntax", and , Functions and Operators.
SELECT
can also be used to retrieve rows computed without reference to any table.
For example:
mysql> SELECT 1 + 1;
-> 2
You are permitted to specify DUAL
as a dummy table name in situations where no tables are referenced:
mysql> SELECT 1 + 1 FROM DUAL;
-> 2
DUAL
is purely for the convenience of people who require that all SELECT
statements should have FROM
and possibly other clauses. MariaDB may ignore the clauses. MariaDB does not require FROM DUAL
if no tables are referenced.
In general, clauses used must be given in exactly the order shown in the syntax description. For example, a HAVING
clause must come after any GROUP BY
clause and before any ORDER BY
clause. The exception is that the INTO
clause can appear either as shown in the syntax description or immediately following the select_expr
list. For more information about INTO
, see , "SELECT ... INTO
Syntax".
The list of select_expr
terms comprises the select list that indicates which columns to retrieve. Terms specify a column or expression or can use *
-shorthand:
- A select list consisting only of a single unqualified
*
can be used as shorthand to select all columns from all tables:SELECT * FROM t1 INNER JOIN t2 ...
can be used as a qualified shorthand to select all columns from the named table:tbl_name
.*SELECT t1.*, t2.* FROM t1 INNER JOIN t2 ...
- Use of an unqualified
*
with other items in the select list may produce a parse error. To avoid this problem, use a qualified
referencetbl_name
.*SELECT AVG(score), t1.* FROM t1 ...
The following list provides additional information about other SELECT
clauses:
- A
select_expr
can be given an alias usingAS
. The alias is used as the expression's column name and can be used inalias_name
GROUP BY
,ORDER BY
, orHAVING
clauses. For example:SELECT CONCAT(last_name,', ',first_name) AS full_name FROM mytable ORDER BY full_name;
The
AS
keyword is optional when aliasing aselect_expr
with an identifier. The preceding example could have been written like this:SELECT CONCAT(last_name,', ',first_name) full_name FROM mytable ORDER BY full_name;
However, because the
AS
is optional, a subtle problem can occur if you forget the comma between twoselect_expr
expressions: MariaDB interprets the second as an alias name. For example, in the following statement,columnb
is treated as an alias name:SELECT columna columnb FROM mytable;
For this reason, it is good practice to be in the habit of using
AS
explicitly when specifying column aliases.It is not permissible to refer to a column alias in a
WHERE
clause, because the column value might not yet be determined when theWHERE
clause is executed. See "Problems with Column Aliases". - The
FROM
clause indicates the table or tables from which to retrieve rows. If you name more than one table, you are performing a join. For information on join syntax, see , "table_references
JOIN
Syntax". For each table specified, you can optionally specify an alias.tbl_name
[[AS]alias
] [index_hint
]The use of index hints provides the optimizer with information about how to choose indexes during query processing. For a description of the syntax for specifying these hints, see , "Index Hint Syntax".
You can use
SET max_seeks_for_key=
as an alternative way to force MariaDB to prefer key scans instead of table scans. See , "Server System Variables".value
- You can refer to a table within the default database as
tbl_name
, or asdb_name
.tbl_name
to specify a database explicitly. You can refer to a column ascol_name
,tbl_name
.col_name
, ordb_name
.tbl_name
.col_name
. You need not specify atbl_name
ordb_name
.tbl_name
prefix for a column reference unless the reference would be ambiguous. See , "Identifier Qualifiers", for examples of ambiguity that require the more explicit column reference forms. - A table reference can be aliased using
ortbl_name
ASalias_name
tbl_name alias_name
:SELECT t1.name, t2.salary FROM employee AS t1, info AS t2 WHERE t1.name = t2.name; SELECT t1.name, t2.salary FROM employee t1, info t2 WHERE t1.name = t2.name;
- Columns selected for output can be referred to in
ORDER BY
andGROUP BY
clauses using column names, column aliases, or column positions. Column positions are integers and begin with 1:SELECT college, region, seed FROM tournament ORDER BY region, seed; SELECT college, region AS r, seed AS s FROM tournament ORDER BY r, s; 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 theORDER BY
clause that you are sorting by. The default is ascending order; this can be specified explicitly using theASC
keyword.If
ORDER BY
occurs within a subquery and also is applied in the outer query, the outermostORDER BY
takes precedence. For example, results for the following statement are sorted in descending order, not ascending order:(SELECT ... ORDER BY a) ORDER BY a DESC;
Use of column positions is deprecated because the syntax has been removed from the SQL standard.
- If you use
GROUP BY
, output rows are sorted according to theGROUP BY
columns as if you had anORDER BY
for the same columns. To avoid the overhead of sorting thatGROUP BY
produces, addORDER BY NULL
:SELECT a, COUNT(b) FROM test_table GROUP BY a ORDER BY NULL;
- MySQL extends the
GROUP BY
clause so that you can also specifyASC
andDESC
after columns named in the clause:SELECT a, COUNT(b) FROM test_table GROUP BY a DESC;
- MySQL extends the use of
GROUP BY
to permit selecting fields that are not mentioned in theGROUP BY
clause. If you are not getting the results that you expect from your query, please read the description ofGROUP BY
found in , "Functions and Modifiers for Use withGROUP BY
Clauses". GROUP BY
permits aWITH ROLLUP
modifier. See , "GROUP BY
Modifiers".- The
HAVING
clause is applied nearly last, just before items are sent to the client, with no optimization. (LIMIT
is applied afterHAVING
.)The SQL standard requires that
HAVING
must reference only columns in theGROUP BY
clause or columns used in aggregate functions. However, MariaDB supports an extension to this behavior, and permitsHAVING
to refer to columns in theSELECT
list and columns in outer subqueries as well.If the
HAVING
clause refers to a column that is ambiguous, a warning occurs. In the following statement,col2
is ambiguous because it is used as both an alias and a column name:SELECT COUNT(col1) AS col2 FROM t GROUP BY col2 HAVING col2 = 2;
Preference is given to standard SQL behavior, so if a
HAVING
column name is used both inGROUP BY
and as an aliased column in the output column list, preference is given to the column in theGROUP BY
column. - Do not use
HAVING
for items that should be in theWHERE
clause. For example, do not write the following:SELECT
col_name
FROMtbl_name
HAVINGcol_name
> 0;Write this instead:
SELECT
col_name
FROMtbl_name
WHEREcol_name
> 0; - The
HAVING
clause can refer to aggregate functions, which theWHERE
clause cannot:SELECT user, MAX(salary) FROM users GROUP BY user HAVING MAX(salary) > 10;
(This did not work in some older versions of MySQL.)
- MySQL permits duplicate column names. That is, there can be more than one
select_expr
with the same name. This is an extension to standard SQL. Because MariaDB also permitsGROUP BY
andHAVING
to refer toselect_expr
values, this can result in an ambiguity:SELECT 12 AS a, a FROM t GROUP BY a;
In that statement, both columns have the name
a
. To ensure that the correct column is used for grouping, use different names for eachselect_expr
. - MySQL resolves unqualified column or alias references in
ORDER BY
clauses by searching in theselect_expr
values, then in the columns of the tables in theFROM
clause. ForGROUP BY
orHAVING
clauses, it searches theFROM
clause before searching in theselect_expr
values. (ForGROUP BY
andHAVING
, this differs from the pre-MySQL 5.0 behavior that used the same rules as forORDER BY
.) - The
LIMIT
clause can be used to constrain the number of rows returned by theSELECT
statement.LIMIT
takes one or two numeric arguments, which must both be nonnegative integer constants, with these exceptions:- Within prepared statements,
LIMIT
parameters can be specified using?
placeholder markers. - Within stored programs,
LIMIT
parameters can be specified using integer-valued routine parameters or local variables.
With two arguments, the first argument specifies the offset of the first row to return, and the second specifies the maximum number of rows to return. The offset of the initial row is 0 (not 1):
SELECT * FROM tbl 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 some large number for the second parameter. This statement retrieves all rows from the 96th row to the last:
SELECT * FROM tbl LIMIT 95,18446744073709551615;
With one argument, the value specifies the number of rows to return from the beginning of the result set:
SELECT * FROM tbl LIMIT 5; # Retrieve first 5 rows
In other words,
LIMIT
is equivalent torow_count
LIMIT 0,
.row_count
For prepared statements, you can use placeholders. The following statements will return one row from the
tbl
table:SET @a=1; PREPARE STMT FROM 'SELECT * FROM tbl LIMIT ?'; EXECUTE STMT USING @a;
The following statements will return the second to sixth row from the
tbl
table:SET @skip=1; SET @numrows=5; PREPARE STMT FROM 'SELECT * FROM tbl LIMIT ?, ?'; EXECUTE STMT USING @skip, @numrows;
For compatibility with PostgreSQL, MariaDB also supports the
LIMIT
syntax.row_count
OFFSEToffset
If
LIMIT
occurs within a subquery and also is applied in the outer query, the outermostLIMIT
takes precedence. For example, the following statement produces two rows, not one:(SELECT ... LIMIT 1) LIMIT 2;
- Within prepared statements,
- A
PROCEDURE
clause names a procedure that should process the data in the result set. For an example, see , "PROCEDURE ANALYSE
", which describesANALYSE
, a procedure that can be used to obtain suggestions for optimal column data types that may help reduce table sizes. - The
SELECT ... INTO
form ofSELECT
enables the query result to be written to a file or stored in variables. For more information, see , "SELECT ... INTO
Syntax". - If you use
FOR UPDATE
with a storage engine that uses page or row locks, rows examined by the query are write-locked until the end of the current transaction. UsingLOCK IN SHARE MODE
sets a shared lock that permits other transactions to read the examined rows but not to update or delete them. See , "SELECT ... FOR UPDATE
andSELECT ... LOCK IN SHARE MODE
Locking Reads".In addition, you cannot use
FOR UPDATE
as part of theSELECT
in a statement such asCREATE TABLE
. (If you attempt to do so, the statement is rejected with the error Can't update table 'new_table
SELECT ... FROMold_table
...old_table
' while 'new_table
' is being created.) This is a change in behavior from MariaDB 5.5 and earlier, which permittedCREATE TABLE ... SELECT
statements to make changes in tables other than the table being created.
Following the SELECT
keyword, you can use a number of options that affect the operation of the statement. HIGH_PRIORITY
, STRAIGHT_JOIN
, and options beginning with SQL_
are MariaDB extensions to standard SQL.
- The
ALL
andDISTINCT
options specify whether duplicate rows should be returned.ALL
(the default) specifies that all matching rows should be returned, including duplicates.DISTINCT
specifies removal of duplicate rows from the result set. It is an error to specify both options.DISTINCTROW
is a synonym forDISTINCT
. HIGH_PRIORITY
gives theSELECT
higher priority than a statement that updates a table. You should use this only for queries that are very fast and must be done at once. ASELECT HIGH_PRIORITY
query that is issued while the table is locked for reading runs even if there is an update statement waiting for the table to be free. This affects only storage engines that use only table-level locking (such asMyISAM
,MEMORY
, andMERGE
).HIGH_PRIORITY
cannot be used withSELECT
statements that are part of aUNION
.STRAIGHT_JOIN
forces the optimizer to join the tables in the order in which they are listed in theFROM
clause. You can use this to speed up a query if the optimizer joins the tables in nonoptimal order.STRAIGHT_JOIN
also can be used in thetable_references
list. See , "JOIN
Syntax".STRAIGHT_JOIN
does not apply to any table that the optimizer treats as aconst
orsystem
table. Such a table produces a single row, is read during the optimization phase of query execution, and references to its columns are replaced with the appropriate column values before query execution proceeds. These tables will appear first in the query plan displayed byEXPLAIN
. See , "Optimizing Queries withEXPLAIN
". This exception may not apply toconst
orsystem
tables that are used on theNULL
-complemented side of an outer join (that is, the right-side table of aLEFT JOIN
or the left-side table of aRIGHT JOIN
.SQL_BIG_RESULT
orSQL_SMALL_RESULT
can be used withGROUP BY
orDISTINCT
to tell the optimizer that the result set has many rows or is small, respectively. ForSQL_BIG_RESULT
, MariaDB directly uses disk-based temporary tables if needed, and prefers sorting to using a temporary table with a key on theGROUP BY
elements. ForSQL_SMALL_RESULT
, MariaDB uses fast temporary tables to store the resulting table instead of using sorting. This should not normally be needed.SQL_BUFFER_RESULT
forces the result to be put into a temporary table. This helps MariaDB free the table locks early and helps in cases where it takes a long time to send the result set to the client. This option can be used only for top-levelSELECT
statements, not for subqueries or followingUNION
.SQL_CALC_FOUND_ROWS
tells MariaDB to calculate how many rows there would be in the result set, disregarding anyLIMIT
clause. The number of rows can then be retrieved withSELECT FOUND_ROWS()
. See , "Information Functions".- The
SQL_CACHE
andSQL_NO_CACHE
options affect caching of query results in the query cache (see , "The MariaDB Query Cache").SQL_CACHE
tells MariaDB to store the result in the query cache if it is cacheable and the value of thequery_cache_type
system variable is2
orDEMAND
.SQL_NO_CACHE
tells MariaDB not to store the result in the query cache.For views,
SQL_NO_CACHE
applies if it appears in anySELECT
in the query. For a cacheable query,SQL_CACHE
applies if it appears in the firstSELECT
of a view referred to by the query.In MariaDB 5.6, these two options are mutually exclusive and an error occurs if they are both specified. Also, these options are not permitted in subqueries (including subqueries in the
FROM
clause), andSELECT
statements in unions other than the firstSELECT
.