Home » Android » android – SQLiteDatabase.query method

android – SQLiteDatabase.query method

Posted by: admin March 11, 2020 Leave a comment

Questions:

I am using the query method of SQLiteDatabase. How do I use the query method?

I tried this:

Cursor cursor = sqLiteDatabase.query(
    tableName, tableColumns, whereClause, whereArgs, groupBy, having, orderBy);

tableColumns – columns parameter is constructed as follows.

String[] columns = new String[]{KEY_ID, KEY_CONTENT};

If we need to get all the fields, how should the column parameter to be constructed. Do we need to include all the Field Names in String array?

How do I properly use the query method?

How to&Answers:

tableColumns

  • null for all columns as in SELECT * FROM ...
  • new String[] { "column1", "column2", ... } for specific columns as in SELECT column1, column2 FROM ... – you can also put complex expressions here:
    new String[] { "(SELECT max(column1) FROM table1) AS max" } would give you a column named max holding the max value of column1

whereClause

  • the part you put after WHERE without that keyword, e.g. "column1 > 5"
  • should include ? for things that are dynamic, e.g. "column1=?" -> see whereArgs

whereArgs

  • specify the content that fills each ? in whereClause in the order they appear

the others

  • just like whereClause the statement after the keyword or null if you don’t use it.

Example

String[] tableColumns = new String[] {
    "column1",
    "(SELECT max(column1) FROM table2) AS max"
};
String whereClause = "column1 = ? OR column1 = ?";
String[] whereArgs = new String[] {
    "value1",
    "value2"
};
String orderBy = "column1";
Cursor c = sqLiteDatabase.query("table1", tableColumns, whereClause, whereArgs,
        null, null, orderBy);

// since we have a named column we can do
int idx = c.getColumnIndex("max");

is equivalent to the following raw query

String queryString =
    "SELECT column1, (SELECT max(column1) FROM table1) AS max FROM table1 " +
    "WHERE column1 = ? OR column1 = ? ORDER BY column1";
sqLiteDatabase.rawQuery(queryString, whereArgs);

By using the Where/Bind -Args version you get automatically escaped values and you don’t have to worry if input-data contains '.

Unsafe: String whereClause = "column1='" + value + "'";
Safe: String whereClause = "column1=?";

because if value contains a ' your statement either breaks and you get exceptions or does unintended things, for example value = "XYZ'; DROP TABLE table1;--" might even drop your table since the statement would become two statements and a comment:

SELECT * FROM table1 where column1='XYZ'; DROP TABLE table1;--'

using the args version XYZ'; DROP TABLE table1;-- would be escaped to 'XYZ''; DROP TABLE table1;--' and would only be treated as a value. Even if the ' is not intended to do bad things it is still quite common that people have it in their names or use it in texts, filenames, passwords etc. So always use the args version. (It is okay to build int and other primitives directly into whereClause though)

Answer:

This is a more general answer meant to be a quick reference for future viewers.

Example

SQLiteDatabase db = helper.getReadableDatabase();

String table = "table2";
String[] columns = {"column1", "column3"};
String selection = "column3 =?";
String[] selectionArgs = {"apple"};
String groupBy = null;
String having = null;
String orderBy = "column3 DESC";
String limit = "10";

Cursor cursor = db.query(table, columns, selection, selectionArgs, groupBy, having, orderBy, limit);

Explanation from the documentation

  • table String: The table name to compile the query against.
  • columns String: A list of which columns to return. Passing null will return all columns, which is discouraged to prevent reading data
    from storage that isn’t going to be used.
  • selection String: A filter declaring which rows to return, formatted as an SQL WHERE clause (excluding the WHERE itself). Passing
    null will return all rows for the given table.
  • selectionArgs String: You may include ?s in selection, which will be replaced by the values from selectionArgs, in order that they
    appear in the selection. The values will be bound as Strings.
  • groupBy String: A filter declaring how to group rows, formatted as an SQL GROUP BY clause (excluding the GROUP BY itself). Passing null
    will cause the rows to not be grouped.
  • having String: A filter declare which row groups to include in the cursor, if row grouping is being used, formatted as an SQL HAVING
    clause (excluding the HAVING itself). Passing null will cause all row
    groups to be included, and is required when row grouping is not being
    used.
  • orderBy String: How to order the rows, formatted as an SQL ORDER BY clause (excluding the ORDER BY itself). Passing null will use the
    default sort order, which may be unordered.
  • limit String: Limits the number of rows returned by the query, formatted as LIMIT clause. Passing null denotes no LIMIT clause.

Answer:

Where clause and args work together to form the WHERE statement of the SQL query. So say you looking to express

WHERE Column1 = 'value1' AND Column2 = 'value2'

Then your whereClause and whereArgs will be as follows

String whereClause = "Column1 =? AND Column2 =?";
String[] whereArgs = new String[]{"value1", "value2"};

If you want to select all table columns, i believe a null string passed to tableColumns will suffice.

Answer:

if your SQL query is like this

SELECT col-1, col-2 FROM tableName WHERE col-1=apple,col-2=mango
GROUPBY col-3 HAVING Count(col-4) > 5  ORDERBY col-2 DESC LIMIT 15;

Then for query() method, we can do as:-

String table = "tableName";
String[] columns = {"col-1", "col-2"};
String selection = "col-1 =? AND col-2=?";       
String[] selectionArgs = {"apple","mango"};
String groupBy =col-3;
String having =" COUNT(col-4) > 5";
String orderBy = "col-2 DESC";
String limit = "15";

query(tableName, columns, selection, selectionArgs, groupBy, having, orderBy, limit);

Answer:

db.query(
        TABLE_NAME,
        new String[] { TABLE_ROW_ID, TABLE_ROW_ONE, TABLE_ROW_TWO },
        TABLE_ROW_ID + "=" + rowID,
        null, null, null, null, null
);

TABLE_ROW_ID + "=" + rowID, here = is the where clause. To select all values you will have to give all column names:

or you can use a raw query like this 
db.rawQuery("SELECT * FROM permissions_table WHERE name = 'Comics' ", null);

and here is a good tutorial for database.