有没有一种方法来检查表是否存在,而不选择和检查它的值?
也就是说,我知道我可以去SELECT testcol FROM testtable并检查返回字段的计数,但似乎必须有一个更直接/优雅的方式来做到这一点。
有没有一种方法来检查表是否存在,而不选择和检查它的值?
也就是说,我知道我可以去SELECT testcol FROM testtable并检查返回字段的计数,但似乎必须有一个更直接/优雅的方式来做到这一点。
当前回答
这里的答案有几个问题需要注意:
1) INFORMATION_SCHEMA。TABLES不包括TEMPORARY表。
2)使用任何类型的SHOW查询,即SHOW TABLES LIKE 'test_table',将强制返回一个结果集到客户端,这是检查表是否存在服务器端的不希望的行为,从存储过程中也返回一个结果集。
3)正如一些用户提到的,你必须小心如何使用SELECT 1 FROM test_table LIMIT 1。
如果你这样做:
SET @table_exists = 0;
SET @table_exists = (SELECT 1 FROM test_table LIMIT 1);
如果表中没有行,则不会得到预期的结果。
下面是一个适用于所有表(甚至是TEMPORARY表)的存储过程。
它可以这样使用:
SET @test_table = 'test_table';
SET @test_db = NULL;
SET @does_table_exist = NULL;
CALL DoesTableExist(@test_table, @test_db, @does_table_exist);
SELECT @does_table_exist;
代码:
/*
p_table_name is required
p_database_name is optional
if NULL is given for p_database_name, then it defaults to the currently selected database
p_does_table_exist
The @variable to save the result to
This procedure attempts to
SELECT NULL FROM `p_database_name`.`p_table_name` LIMIT 0;
If [SQLSTATE '42S02'] is raised, then
SET p_does_table_exist = 0
Else
SET p_does_table_exist = 1
Info on SQLSTATE '42S02' at:
https://dev.mysql.com/doc/refman/5.7/en/server-error-reference.html#error_er_no_such_table
*/
DELIMITER $$
DROP PROCEDURE IF EXISTS DoesTableExist
$$
CREATE PROCEDURE DoesTableExist (
IN p_table_name VARCHAR(64),
IN p_database_name VARCHAR(64),
OUT p_does_table_exist TINYINT(1) UNSIGNED
)
BEGIN
/* 793441 is used in this procedure for ensuring that user variables have unique names */
DECLARE EXIT HANDLER FOR SQLSTATE '42S02'
BEGIN
SET p_does_table_exist = 0
;
END
;
IF p_table_name IS NULL THEN
SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'DoesTableExist received NULL for p_table_name.';
END IF;
/* redirect resultset to a dummy variable */
SET @test_select_sql_793441 = CONCAT(
"SET @dummy_var_793441 = ("
" SELECT"
" NULL"
" FROM ",
IF(
p_database_name IS NULL,
"",
CONCAT(
"`",
REPLACE(p_database_name, "`", "``"),
"`."
)
),
"`",
REPLACE(p_table_name, "`", "``"),
"`"
" LIMIT 0"
")"
)
;
PREPARE _sql_statement FROM @test_select_sql_793441
;
SET @test_select_sql_793441 = NULL
;
EXECUTE _sql_statement
;
DEALLOCATE PREPARE _sql_statement
;
SET p_does_table_exist = 1
;
END
$$
DELIMITER ;
其他回答
扩展这个答案,可以进一步编写一个函数,根据表是否存在返回TRUE/FALSE:
CREATE FUNCTION fn_table_exists(dbName VARCHAR(255), tableName VARCHAR(255))
RETURNS BOOLEAN
BEGIN
DECLARE totalTablesCount INT DEFAULT (
SELECT COUNT(*)
FROM information_schema.TABLES
WHERE (TABLE_SCHEMA COLLATE utf8_general_ci = dbName COLLATE utf8_general_ci)
AND (TABLE_NAME COLLATE utf8_general_ci = tableName COLLATE utf8_general_ci)
);
RETURN IF(
totalTablesCount > 0,
TRUE,
FALSE
);
END
;
SELECT fn_table_exists('development', 'user');
这是我的“去”EXISTS过程,检查临时表和正常表。此过程适用于MySQL 5.6及以上版本。@DEBUG参数是可选的。默认模式是假设的,但是可以在@s语句中连接到表。
drop procedure if exists `prcDoesTableExist`;
delimiter #
CREATE PROCEDURE `prcDoesTableExist`(IN pin_Table varchar(100), OUT pout_TableExists BOOL)
BEGIN
DECLARE `boolTableExists` TINYINT(1) DEFAULT 1;
DECLARE CONTINUE HANDLER FOR 1243, SQLSTATE VALUE '42S02' SET `boolTableExists` := 0;
SET @s = concat('SELECT null FROM `', pin_Table, '` LIMIT 0 INTO @resultNm');
PREPARE stmt1 FROM @s;
EXECUTE stmt1;
DEALLOCATE PREPARE stmt1;
set pout_TableExists = `boolTableExists`; -- Set output variable
IF @DEBUG then
select IF(`boolTableExists`
, CONCAT('TABLE `', pin_Table, '` exists: ', pout_TableExists)
, CONCAT('TABLE `', pin_Table, '` does not exist: ', pout_TableExists)
) as result;
END IF;
END #
delimiter ;
下面是@debug on的示例调用语句:
set @DEBUG = true;
call prcDoesTableExist('tempTable', @tblExists);
select @tblExists as '@tblExists';
变量@tblExists返回一个布尔值。
SELECT count(*)
FROM information_schema.TABLES
WHERE (TABLE_SCHEMA = 'your_db_name') AND (TABLE_NAME = 'name_of_table')
如果得到非零计数,则表存在。
在创建TABLE之前,最好先检查SQL Server数据库中是否存在该表。
USE [DB_NAME]
GO
IF OBJECT_ID('table_name', 'U') IS NOT NULL
BEGIN
PRINT 'Table exists.'
END
ELSE
BEGIN
PRINT 'Table does not exist.'
END
另外 使用sys。对象,检查SQL Server中是否存在表。
USE [DB_NAME]
GO
IF EXISTS(SELECT 1 FROM sys.Objects
WHERE Object_id = OBJECT_ID(N'table_name')
AND Type = N'U')
BEGIN
PRINT 'Table exists.'
END
ELSE
BEGIN
PRINT 'Table does not exist.'
END
上述修改后的解决方案不需要明确了解当前数据库。这样就更灵活了。
SELECT count(*) FROM information_schema.TABLES WHERE TABLE_NAME = 'yourtable'
AND TABLE_SCHEMA in (SELECT DATABASE());