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12.2.9. `SELECT` Syntax

12.2.9.1. JOIN Syntax
12.2.9.2. Index Hint Syntax
12.2.9.3. UNION Syntax

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

SELECT is used to retrieve rows selected from one or more tables, and can include UNION statements and subqueries. See Section 12.2.9.3, “UNION Syntax”, and Section 12.2.10, “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 one select_expr.
table_references indicates the table or tables from which to retrieve rows. Its syntax is described in Section 12.2.9.1, “JOIN Syntax”.
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 no WHERE clause.

In the WHERE clause, you can use any of the functions and operators that MySQL supports, except for aggregate (summary) functions. See Chapter 11, 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 allowed 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. MySQL may ignore the clauses. MySQL 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.

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 ...
```
tbl_name.* can be used as a qualified shorthand to select all columns from the named table:
```
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 tbl_name.* reference
```
SELECT AVG(score), t1.* FROM t1 ...
```

The following list provides additional information about other SELECT clauses:

A select_expr can be given an alias using AS alias_name. The alias is used as the expression's column name and can be used in GROUP BY, ORDER BY, or HAVING 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 a select_expr. 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 two select_expr expressions: MySQL 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 allowable to refer to a column alias in a WHERE clause, because the column value might not yet be determined when the WHERE clause is executed. See Section B.5.5.4, “Problems with Column Aliases”.
The FROM table_references 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 Section 12.2.9.1, “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 Section 12.2.9.2, “Index Hint Syntax”.

You can use SET max_seeks_for_key=value as an alternative way to force MySQL to prefer key scans instead of table scans. See Section 5.1.4, “Server System Variables”.
You can refer to a table within the default database as tbl_name, or as db_name.tbl_name to specify a database explicitly. 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 unless the reference would be ambiguous. See Section 8.2.1, “Identifier Qualifiers”, for examples of ambiguity that require the more explicit column reference forms.

A table reference can be aliased using tbl_name AS alias_name or 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 and GROUP 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 the ORDER BY clause that you are sorting by. The default is ascending order; this can be specified explicitly using the ASC keyword.

If ORDER BY occurs within a subquery and also is applied in the outer query, the outermost ORDER 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 the GROUP BY columns as if you had an ORDER BY for the same columns. To avoid the overhead of sorting that GROUP BY produces, add ORDER 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 specify ASC and DESC 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 allow selecting fields that are not mentioned in the GROUP BY clause. If you are not getting the results that you expect from your query, please read the description of GROUP BY found in Section 11.12, “Functions and Modifiers for Use with GROUP BY Clauses”.
GROUP BY allows a WITH ROLLUP modifier. See Section 11.12.2, “GROUP BY Modifiers”.
The HAVING clause is applied nearly last, just before items are sent to the client, with no optimization. (LIMIT is applied after HAVING.)

The SQL standard requires that HAVING must reference only columns in the GROUP BY clause or columns used in aggregate functions. However, MySQL supports an extension to this behavior, and allows HAVING to refer to columns in the SELECT 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 in GROUP BY and as an aliased column in the output column list, preference is given to the column in the GROUP BY column.
Do not use HAVING for items that should be in the WHERE clause. For example, do not write the following:
```
SELECT col_name FROM tbl_name HAVING col_name > 0;
```
Write this instead:
```
SELECT col_name FROM tbl_name WHERE col_name > 0;
```
The HAVING clause can refer to aggregate functions, which the WHERE 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 allows 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 MySQL also allows GROUP BY and HAVING to refer to select_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 each select_expr.
MySQL resolves unqualified column or alias references in ORDER BY clauses by searching in the select_expr values, then in the columns of the tables in the FROM clause. For GROUP BY or HAVING clauses, it searches the FROM clause before searching in the select_expr values. (For GROUP BY and HAVING, this differs from the pre-MySQL 5.0 behavior that used the same rules as for ORDER BY.)
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 both be nonnegative integer constants (except when using prepared statements).

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 row_count is equivalent to 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, MySQL also supports the LIMIT row_count OFFSET offset syntax.

If LIMIT occurs within a subquery and also is applied in the outer query, the outermost LIMIT takes precedence. For example, the following statement produces two rows, not one:
```
(SELECT ... LIMIT 1) LIMIT 2;
```
A PROCEDURE clause names a procedure that should process the data in the result set. For an example, see Section 22.4.1, “PROCEDURE ANALYSE”, which describes ANALYSE, a procedure that can be used to obtain suggestions for optimal column data types that may help reduce table sizes.
The SELECT ... INTO OUTFILE 'file_name' form of SELECT writes the selected rows to a file. The file is created on the server host, so you must have the FILE privilege to use this syntax. file_name cannot be an existing file, which among other things prevents files such as /etc/passwd and database tables from being destroyed. The character_set_filesystem system variable controls the interpretation of the file name.

The SELECT ... INTO OUTFILE statement is intended primarily to let you very quickly dump a table to a text file on the server machine. If you want to create the resulting file on some client host other than the server host, you cannot use SELECT ... INTO OUTFILE. In that case, you should instead use a command such as mysql -e "SELECT ..." > file_name to generate the file on the client host.

SELECT ... INTO OUTFILE is the complement of LOAD DATA INFILE. Column values are written converted to the character set specified in the CHARACTER SET clause. If no such clause is present, values are dumped using the binary character set. In effect, there is no character set conversion. If a table contains columns in several character sets, the output data file will as well and you may not be able to reload the file correctly.

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 12.2.6, “LOAD DATA INFILE Syntax”, for information about the FIELDS and LINES clauses, including their default values and allowable values.

FIELDS ESCAPED BY controls how to write special characters. If the FIELDS ESCAPED BY character is not empty, it is used as a prefix that precedes following characters on output:
- The FIELDS ESCAPED BY character
- The FIELDS [OPTIONALLY] ENCLOSED BY character
- The first character of the FIELDS TERMINATED BY and LINES TERMINATED BY values
- ASCII NUL (the zero-valued byte; what is actually written following the escape character is ASCII “0”, not a zero-valued byte)
The FIELDS TERMINATED BY, ENCLOSED BY, ESCAPED BY, or LINES TERMINATED BY characters must be escaped so that you can read the file back in reliably. ASCII NUL is escaped to make it easier to view with some pagers.

The resulting file does not have to conform to SQL syntax, so nothing else need be escaped.

If the FIELDS ESCAPED BY character is empty, no characters are escaped and NULL is output as NULL, not \N. It is probably not a good idea to specify an empty escape character, particularly if field values in your data contain any of the characters in the list just given.

Here is an example that produces a file in the comma-separated values (CSV) format used by many programs:
```
SELECT a,b,a+b INTO OUTFILE '/tmp/result.txt'
  FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"'
  LINES TERMINATED BY '\n'
  FROM test_table;
```
If you use INTO DUMPFILE instead of INTO OUTFILE, MySQL writes only one row into the file, without any column or line termination and without performing any escape processing. This is useful if you want to store a BLOB value in a file.
Note

Any file created by INTO OUTFILE or INTO DUMPFILE is writable by all users on the server host. The reason for this is that the MySQL server cannot create a file that is owned by anyone other than the user under whose account it is running. (You should never run mysqld as root for this and other reasons.) The file thus must be world-writable so that you can manipulate its contents.

If the secure_file_priv system variable is set to a nonempty directory name, the file to be written must be located in that directory.
The INTO clause can name a list of one or more variables, which can be user-defined variables, or parameters or local variables within a stored function or procedure body (see Section 12.7.3.3, “SELECT ... INTO Statement”). The selected values are assigned to the variables. The number of variables must match the number of columns. The query should return a single row. If the query returns no rows, a warning with error code 1329 occurs (No data), and the variable values remain unchanged. If the query returns multiple rows, error 1172 occurs (Result consisted of more than one row). If it is possible that the statement may retrieve multiple rows, you can use LIMIT 1 to limit the result set to a single row.

In the context of such statements that occur as part of events executed by the Event Scheduler, diagnostics messages (not only errors, but also warnings) are written to the error log, and, on Windows, to the application event log. For additional information, see Section 18.4.5, “Event Scheduler Status”.
The SELECT syntax description at the beginning this section shows the INTO clause near the end of the statement. It is also possible to use INTO immediately following the select_expr list.
An INTO clause should not be used in a nested SELECT because such a SELECT must return its result to the outer context.
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. Using LOCK IN SHARE MODE sets a shared lock that allows other transactions to read the examined rows but not to update or delete them. See Section 13.6.8.3, “SELECT ... FOR UPDATE and SELECT ... LOCK IN SHARE MODE Locking Reads”.

Following the SELECT keyword, you can use a number of options that affect the operation of the statement.

The ALL, DISTINCT, and DISTINCTROW options specify whether duplicate rows should be returned. If none of these options are given, the default is ALL (all matching rows are returned). DISTINCT and DISTINCTROW are synonyms and specify removal of duplicate rows from the result set.

HIGH_PRIORITY, STRAIGHT_JOIN, and options beginning with SQL_ are MySQL extensions to standard SQL.

HIGH_PRIORITY gives the SELECT 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. A SELECT 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 as MyISAM, MEMORY, and MERGE).

HIGH_PRIORITY cannot be used with SELECT statements that are part of a UNION.
STRAIGHT_JOIN forces the optimizer 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 optimizer joins the tables in nonoptimal order. STRAIGHT_JOIN also can be used in the table_references list. See Section 12.2.9.1, “JOIN Syntax”.

STRAIGHT_JOIN does not apply to any table that the optimizer treats as a const or system 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 by EXPLAIN. See Section 7.2.1, “Optimizing Queries with EXPLAIN”. This exception may not apply to const or system tables that are used on the NULL-complemented side of an outer join (that is, the right-side table of a LEFT JOIN or the left-side table of a RIGHT JOIN.
SQL_BIG_RESULT can be used with GROUP BY or DISTINCT to tell the optimizer that the result set has many rows. In this case, MySQL directly uses disk-based temporary tables if needed, and prefers sorting to using 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. This option can be used only for top-level SELECT statements, not for subqueries or following UNION.
SQL_SMALL_RESULT can be used with GROUP BY or DISTINCT to tell the optimizer that the result set is small. In this case, MySQL uses fast temporary tables to store the resulting table instead of using sorting. This should not normally be needed.
SQL_CALC_FOUND_ROWS 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 11.11.3, “Information Functions”.
The SQL_CACHE and SQL_NO_CACHE options affect caching of query results in the query cache (see Section 7.5.5, “The MySQL Query Cache”). SQL_CACHE tells MySQL to store the result in the query cache if it is cacheable and the value of the query_cache_type system variable is 2 or DEMAND. SQL_NO_CACHE tells MySQL not to store the result in the query cache.

For views, SQL_NO_CACHE applies if it appears in any SELECT in the query. For a cacheable query, SQL_CACHE applies if it appears in the first SELECT of a view referred to by the query.

As of MySQL 5.5.3, these two options are mutually exclusive and an error occurs if they are both specified. Also, these options are disallowed in subqueries (including subqueries in the FROM clause, and SELECT statements in unions other than the first SELECT.

Before MySQL 5.5.3, for a query that uses UNION or subqueries, the following rules apply:
- SQL_NO_CACHE applies if it appears in any SELECT in the query.
- For a cacheable query, SQL_CACHE applies if it appears in the first SELECT of the query.

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Posted by Colin Nelson on February 26 2003 12:10am

12.2.9. SELECT Syntax

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12.2.9. `SELECT` Syntax