Understanding NULL in SQL In SQL, NULL signifies the absence of information or a missing value. This is different from a zero value or a field that contains spaces. A NULL value is a field with no value; it is not equivalent to zero or a space, which are actual values. It's crucial...
1.null在in中还是在not in中都找不到任何数据,这一点很重要,经常适用(not) in 查询的时候要注意保证该列不含有null值,否则将导致如果有null值存在,查询不到任何数据;最好的方法是加个条件is not null的条件在子查询中; 2.利用count进行计算行数的时候,对指定字段的不会将null值计算在内,如果需要将null值的...
Transact-SQL also offers an extension for null processing. If the option ANSI_NULLS is set to OFF, comparisons between nulls, such as NULL = NULL, evaluate to TRUE. Comparisons between NULL and any data value evaluate to FALSE. See Also Reference AND (Transact-SQL) NOT (Transact-SQL) ...
IF(predicate, true-value, false-value)如果谓词为 true，则 IF 计算为 true 值，或在下面的查询中...
plsql_sentence endif; 1. 2. 3. CASE语句 CASE 语句也是用来进行条件判断的,它提供了多个条件进行选择,可以实现比 IF 语句更复杂的条件判断。CASE 语句的基本形式如下: CASE case_value WHEN when_value THEN statement_list [WHEN when_value THEN statement_list]... ...
同事说查询遇到一个奇怪的事,2个表进行not in 操作没有返回结果,正常情况下应该是有返回的。 一.问题重现 一般来说,问题能重现就是好消息,最怕不能重现。 SQL> connscott/tiger; Connected. SQL> descemp Name Null? Type --- --- EMPNO NOT NULLNUMBER(4) ENAME VARCHAR2(10) JOB VARCHAR2(9) MGR ...
A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. If you specify a prefix value for a column in a UNIQUE index, the column values must be unique within...
SqlType.Null 所有System.Data.SqlTypes数据类型都实现INullable。 如果可使用隐式强制转换运算符将强类型的 null 值转换为列的数据类型,则应执行赋值。 否则,将引发无效强制转换异常。 null 如果“null”是给定DataColumn数据类型的合法值,则会将其强制转换为与DbNull.Value类型 (Null) 关联的相应INullable或SqlType...
(nRow);// Initialize variables to use when// extracting the data.SqlBoolean isColumnNull =false; SqlInt32 idValue = SqlInt32.Zero; SqlString descriptionValue = SqlString.Null;// Iterate through the DataTable and display the values.foreach(DataRow rowintable.Rows) {// Assign values to ...
Re: how do you insert a null value in a parameterized SQL statement? System.DBNull.Value can't be converted to a string it will give you a conversion error. Dealing with DBNulls are annoying, but you have no choice really when dealing with databases. Reply With Quote May...