在JAVA for MySQL PreparedStatment中绑定NULL？ [英] Binding NULL in JAVA for MySQL PreparedStatment?

问题描述

MySQL使用IS NULL而不是NULL，所以我如何绑定变量来实现相同？

MySQL uses "IS NULL" instead of "NULL" so how do i bind variable to achieve the same ?

现在我试过这个

selectStmt.setNull( 2, java.sql.Types.VARCHAR );

如果是这样，在预备状态下如何设置IS NULL？

when i printed "java.sql.Types.VARCHAR" shows value = 12 ? If so how does this set " IS NULL" in preparedstatment ?

更多详细信息：

即我有兴趣查询（MySQL）为null，如果我有输入中的字符串=。所以逻辑就是这样，

i.e I am interested in querying (MySQL) for null if i have string = " " in input. So logic goes like this,

Connection c = SQLCon.connect();
PreparedStatement selectStmt = c.prepareStatement( "select * from "
                                    + TableName
                                    + " where P = ? AND M = ? AND PR = ? AND DR = ? " );


if ( P.trim().equals( "" ) ) 
{
   selectStmt.setNull( 1, java.sql.Types.VARCHAR );
} else 
{
   selectStmt.setString( 1, P );
}

if ( M.trim().equals( "" ) ) 
{
   selectStmt.setNull( 2, java.sql.Types.VARCHAR );
} else 
{
   selectStmt.setString( 2, M );
}

请咨询

推荐答案

如果要匹配where子句中可能为null的列与可能为null的参数匹配，则实际上需要将其绑定两次，因为在SQL中为null不等于其他任何东西，甚至是另一个null。

If you want to match a column in a where clause that might be null with a parameter that might be null, you actually need to bind it twice, because in SQL a null doesn't equal anything else, even another null.

PreparedStatement stmt = conn.prepareStatement("select * from foo where (? is null and bar is null) or (bar = ?)");
col = 1;
setVarchar(stmt, col++, var);
setVarchar(stmt, col++, var);

void setVarchar(PreparedStatement stmt, int col, String var)
{
  if (var == null)
    stmt.setNull(col, java.sql.Types.VARCHAR);
  else
    stmt.setString(col, var);
}

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